Will President Reagan think they are freedom fighters, too? Will overt and covert U.S. funds be sent to aid the 22 million black South Africans when and if they move against their government? The rush- hour explosion of a car bomb in Pretoria, which killed 18 people and injured nearly 200 others, makes it necessary to ask these questions now.

You can bet we're going to be on one side or the other if war breaks out. Don't forget that South Africa's natural resources and American investments there are vital to our well-being and national security. We've been told that for years by our government. Its policies have been squarely based on the necessity for a continuing relationship. Administration upon administration has worked vigorously to ensure that U.S. citizens' concerns about that relationship have stayed rhetorical. There's not a nick in the cord that binds us to the government of Grand Apartheid.

With slight alterations in Hitler's concept of a master race and modifications of his "Final Solution" procedures, the government of South Africa, in 1948, adopted and put into effect a plan to keep all power in its white hands:

* All non-whites disenfranchised.

* Eighty-six percent of the land for whites only (one-fifth of today's population).

* Fourteen percent of the worst land for blacks (state designated: "homelands" or "reserves").

* Elimination of "black spots" (the government name for places where black people live) by forcibly removing people, the dead as well as the living.

* 4 Pass laws to aid "influx control," under which every South African black must carry a kind of interior passport to be at the grocery store, work, anywhere, even at home. (South Africa's highest court yesterday said the pass rules must be liberalized, but what the government will do is unclear.)

* No real South African passports for blacks who have received "homeland" assignments. "Homelands" are make-believe "nations," and the South African government expects them to issue passports, which no other nation recognizes or respects.

Foreigners of no standing in their own country, trapped there like mules in a briar fence, but treated more cruelly in every way, black people resist. For 35 years that resistance has risen and fallen back so often that our government merely stews around, groping for some inoffensive statement to issue in times of crisis, and remains inert when things cool down.

Oh, even this administration recites the litany of repugnance toward apartheid, also deplores it, when pressed. But, by and large, it only hopes that the South Afri- can government will muddle through and find some solution to its "problem." Meanwhile, "we're hoping right along with you, black folks," is what the United States offers to the enthralled majority of South Africans.

In each year of these 31/2 decades, though, new flashpoints have been added. There are now so many anniversaries of ghastly events, so many leaders lost to mourn. There is such a body of suffering and deliberate deprivation that South Africans, black and white, freeze with dread as a significant date comes closer and at each new incident. They wait to see if this is the beginning of the end.

Could the exploding car trigger a chain of events leading to massive death and destruction? Or does it signal a long reign of guerrilla terror?

Whatever it was, a trigger, a signal or a one-time incident, people died and a new flashpoint has been added. This is another moment of immense danger to the people of South Africa.

When and if 22 million black citizens rise against the 5 million white citizens, it should not come as a surprise to anyone in this country. This will not be an occasion to fly by the seat of Ronald Reagan's pants. No acting first and explaining it all later. There is a bona-fide issue here, and this democracy has got to know in advance what course it will take. There is division on the issue now.

What isn't known is if the U.S. majority opposed to apartheid will prevail because we don't know where this administration stands or what action it might take to plunge us into a crisis on the wrong side of the issue, or what action the majority is willing to take.

If the Pretoria explosion was a once-only happening, a clearly stated American position could cause the government of South Africa to move and move fast to end apartheid and establish a democratic state.

If anything was ever worth the effort, this is it.