It sounds like you're using a product in/on your car that is attracting them. One that I know from experience as being a spider magnet is Meguiar's Ultimate Wash and Wax (the fluorescent yellow stuff). When I used to leave some of the residue in the bottom of my wash bucket every week no joke EVERY SINGLE following week I would find at least one spider in there that died because it went in looking for food and couldn't get back out. The wash did smell like bananas lol.

 

Hmm, interesting theory! We might be on to something then!