LAST DECEMBER, we opposed an attempt in Congress to veto a Federal Trade Commission rule requiring used-car dealers to put stickers on cars listing any defects they know about. The issue never came to a vote in 1981 and is before Congress again. We continue to believe that the FTC rule is a very mild attempt to bring a measure of respectability to a market with a reputation for shoddy dealing. Opponents of the rule argue that it would increase the price of used cars sold by dealers by an amount far beyond the value any warnings would give consumers.

Some opponents of the rule, however, do not seem to have much confidence in the ability of ordinary voters to understand their arguments. At least this is what we conclude from the highly unusual refusal of the chief sponsor of the veto resolution in the House, Rep. Gary Lee (R-N.Y.), to name the 200-plus cosponsors he claims to have. (His counterpart in the Senate, Larry Pressler of South Dakota, has listed his 42 cosponsors.)

An aide to Rep. Lee says that to name the cosponsors would provide Congress Watch and other supporters of the FTC rule with a handy target list, against which they could use simplistic, ad hominem arguments.

What kind of arguments? The kind Congress Watch makes when it points out that:

* 16 congressman cosponsored the veto resolution in 1981 within 10 days of receiving campaign contributions from the National Automobile Dealers Association, which opposes the FTC rule.

* 18 of 20 representatives who received $1,000 or more in campaign contributions after the FTC issued its rule supported the veto resolution.

* 58 of 71 representatives receiving NADA campaign contributions after the FTC issued its rule supported the veto resolution.

There is no proof that members of Congress adopted the NADA position in return for these contributions. But the timing and circumstances do raise questions that constituents--and opponents-- may want to have answered.

Disclosure of campaign contributions is required so that voters can ask such questions. Congressman Lee's refusal to name his cosponsors tends to frustrate the aim ofthese important laws. It may be difficult for congressmen supporting the veto resolution to explain their position to their constituents, but such difficulty surely goes with the territory. There is a wonderful irony about refusal to disclose the names of the 200 cosponsors of a resolution that prevent disclosure of defects in used cars.