Well, I can think of two reasons why it might not work. Dunno if they’ll help.
One, are you close enough to both your parts and where the welder is installed on your vehicle so that they both count for crafting (also your glare protection)? That would be the first thing to check – I usually put my welder diagonal from my car door so I can stand in the doorway and therefore be within reach of everything. It sometimes doesn’t count parts if they’re obscured by ‘board’ vehicle tiles (IE: out of LOS).
Or, two, are you just (a)ctivating the welder? Don’t do that. Just (e)xamine your vehicle in a position where you can reach everything and (i)nstall the item you want.
Now, if those two aren’t your problem, then I suggest going into the vehicle menu, selecting what you want to install, and looking at the ‘recipe’ to see which thing you’re missing. That should give you some information and will give us a better idea of why you can’t craft.