Push the F-key that corresponds to the squad member and then push the 6 key on your number row. This is the actions menu. It's usually the 1 key to repair something or heal something.

yea I guess you can only repair the vehicle once it gets badly damged. I tried repairing it with just a red engine, and it wouldnt work. It works when the vehicle is badly damaged

If it's only a little bit damaged, you can take it to the main base and use the vehicle refit support to restore it to good condition. The repair specialist is mainly for more significantly damaged vehicles. I have used it to repair a crashed helicopter-taxi helicopter before, which wasn't too damaged, and was able to successfully fly the helicopter (I may have done a vehicle refit as well). Repair specialists can also get quite useful late in the game, when there are a lot of damaged vehicles around.