Many homeowners presently are enjoying a convenience that enables them to automatically open the garage door from inside the car as it moves up the driveway. This device not only eliminates the need to get out of the car on a rainy or windy night to open the garage door, but it is also an important safety feature -- especially for women who come home alone at night.

However, as with many other luxury, the cost of having the device installed professionally could prove prohibitive. (Labor alone often accounts for 30 percent to 50 percent of the total cost.) To bring prices down, some manufacturers have redesigned these units to simplify the job of installation for the do-it-yourselfer.

Now the two largest makers of these devices -- the Alliance Manufacturing Co., Lake Park Blvd., Alliance, Ohio (manufacturer of Genie Door Openers), and the Chamberlain Manufacturing Corp., 845 Larch Ave., Elmhurst, Ill. 60126 -- are selling easy-to-install, prepackaged partially assembled garage-door openers with full installation instructions that any reasonably handy homeowner should have no trouble following.

In each case the 10-foot-long rail that encloses the drive chain or screw mechanism comes with the long rail in sections that can be put together at home. The entire unit fits into a carton that can be easily carried home in an ordinary car trunk.

(Automatic garage-door openers made several years ago were bulky and hard for the dealer to store or display, and not easily carried home in the average car.)

Chamberlain's new Heavy Duty 555 is a chain-driven model with a four-section T-bar rail that serves as a guide for the drive chain as it travels around the drive sprockets and trolley assembly. This model, complete with radio transmitter for the car, manual door button, and the built-in receiver that activates the motor, costs about $200.

It also comes with a unique safety feature: a photocell sensor and an invisible light-beam projector that is installed about 6 inches above the floor. When the light beam is interrupted -- for example, by a child or a small animal -- it automatically will stop and reverse the downward travel of the door before the door touches the child or animal. And, like most other garage-door openers, the door also will retract automatically if it actually hits an obstruction. (Chamberlain also makes chain-drive models, but at prsent these do not have the new electric-eye feature.)

The Genie garage-door openers have a screw-drive mechanism instead of a chain drive (the company makes both chain drives and screw drives but their screw-drive units are more popular). These also have a new safety feature: You must push the button on the transmitter twice to make the door start closing, thus minimizing the danger of hitting the button accidentally. If the door strikes an obstruction on the way down, it reverses automatically. The button is pushed only once to open the door.

Three models are available, ranging from a little over $100 for at least expensive chain-drive model to about $180 for the largest screw-drive model (the most popular is the mid-size, screw-drive model GS-800, which cost from $140 to $160).

All these units -- from Alliance, Chamberlain and other companies such as Stanley and Sears -- come with radio-operated transmitters and receivers that can be coded so the signal used will not conflict with others in the neighborhood, and so the receiver will not be accidentally triggered by stray signals from planes or other electonic equipment in the area.

Although installation procedures vary, depending on the brand and on the type of garage door (sectional, one-piece with track or one-piece without track), the first step is making certain the door is properly balanced so it moves up and down easily without binding or sticking.

To test for proper balance, raise the door about 3 feet off the ground, then release it. The door should remain in place without falling or creeping upward. If it doesn't, the springs need adjusting, or the rolling and moving parts need lubrication. Check to see that all mounting bolts are tight on the rollers attached to the door, as well as on the supporting brackets that hold the tracks in place against the overhead beams or ceiling.

The next step is to assemble the unassembled rail and drive mechanism that will connect the motor unit to the top of the door. Then, as shown in the accompanying illustration of a Genie unit being installed, the next step is mounting the power unit to the ceiling of the garage.

With the Genie, this is the one part of the job that requires some individual planning on the part of the do-it-yourselfer: constructing a suitable platform for mounting the power unit overhead. This can be done by building a small wood platform (something like an upside-down table), or by fabricating a mounting platform from angle iron or pieces of electrical conduit (3/4-inch steel tubing). With the Chamberlain door opener a mounting kit includes all the metal brackets needed to mount the power unit against the ceiling.

After the mounting platform has been attached to the ceiling, the far end of the assembled 10-foot-long rail (along which the drive screw or drive chain travels) is fastened to the door jamb directly above the center line of the door, then the rail and power unit is swung up in such a away that the power unit can be fastened to the previously prepared mounting platform on the ceiling.

Instructions furnished with the kit tell how the pivot arm that connects the drive mechanism to the top of the door is attached to the door near its highest point. The up-limit and down-limit switches are positioned next and adjusted for proper opening and closing limits, after which the manual push button (which enables you to open the door from inside the garage) is installed in a convenient location and connected to the power unit with low-voltage bell wire.

The battery-operated transmitter requires no wiring or other installation inside the car other than affixing it to the dashboard with adhesive strips or Velcro.