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