My 87 SuperCab had this problem and the gasket and all others in the jy that I found were hardened and cracked. I had a partial tube of butyl rubber (black) for window sealing around, so I put on a pair of mechanics nitrile gloves, squirted a generous amount of sealer into my hand and massaged it into all the cracks in my old gasket being careful not to break it. Then I wiped most of the excess off with clean gloves and installed the light with the gasket. Once installed, I used another clean glove and my finger to smooth the gasket's outer surface. Works great and you can't tell unless you get right up to it and look closely.