on my 91 if i buy a daytime running light module off ebay can i plug it in and have daylight running lights or is it more complicated than that?

 

It kind of depends on the module. The original DRLs would run the high beams at half-power when the car is "on" and the lights are not. You can do it yourself by splicing a relay and high power resistor into the high beam wires of the headlight harness.

But why bother? If you're worried about being visible during the day, why not just turn on your head lights?