I want to make some copper trim for a car. I know copper corrodes galvanized pipes, what about sheet metal?

Question

copper wire grounded to the frame isn't a problem, but it's not exposed to the elements!

Answer 1

Dissimilar metals will almost immediately begin the galvanic corrosion cycle... some sort of gasket, tape, etc. must be used to isolate the two materials.

Answer 2

Same problem with sheet metal.  Copper is anodic to steel (or maybe its the other way around).  Either way, mixing copper, steel, and oh say rain water would be very bad for your car.  If you really want that copper you could try some sort of insulator between them.  Sticking the copper on with trim tape would probably be o.k. just as long as you are sure the copper isn't touching unpainted steel.