While I am sure there is a diagnostic in the FSM, the first thing I would do is pull back the flexible boots between the doors and the body, on both sides. Look for ANY broken wires. This is a common problem, causing all sorts of electrical problems. My advice to you is to get an FSM if you intend to keep your Riviera.
_________________
98 Riviera SC3800 All stock except gutted air box.
1970 Buick GS455 Stage1, TSP built 470BBB, 602HP/589TQ
Best MPH, 116.06 MPH, Best ET, 11.54
https://www.youtube.com/watch?v=UHCda-t_Jls
https://www.youtube.com/watch?v=sfT2tEO4XcU