I have a 1988 G-20 van with dual headlights. Today after I changed the upper headlight on the drivers side it gets dim when I put the brights on. Anyone have a clue why?

 

You bought a bad one? It does happen. You might check the plug for corrosion. But usually if it's corroded, it will work or it won't.

 

bad ground can steal voltage. Check all the connections from the relay forward.

 

I have no idea where the headlight relay is.

 

IFAIK, none of the G vans from the 70s to 1995 had a headlight relay. None of mine did until I rewired the lights (on my '79) and added 2 of them.

I would suspect a bad bulb, connector, or ground wire. The connector or ground wire could have been marginal before you touched anything, and moving things around could have finished them off.

I have found the driver side headlights have more problems with connectors than the passenger side, possibly because they are very close to the battery and the acidic vapors it generates.