Trial and error and observation is how most people start. If that doesn't get a result, you will need to step it up. A multi-meter or test light are cheap and necessary if you want to make sense of whats going wrong electrically. And get a download of the FSM (factory service manual AKA BGB), its a detailed guide to what can go wrong and how to test it.
Older cars can have poor earths or power feeds due to corrosion, loosening of connections, wire breaks, insulation breaks or previous owners mods. For our 80's and 90's cars, some of the electrical path might be on a circuit board, tracks across plastic ribbon with rivets, solder and plug joints. All of these can fail too.
Pressing or tapping a suspect area, disconnecting and reconnecting plugs and carefully looking for cracks or other tells might show up an intermittent fault. Start with the wear and tear areas. If nothing obvious is wrong, then a multimeter will let you confirm that power is arriving where it should be and that it can continue to ground.
Adding this diagnostic info (is there power?, is there continuity to earth?) will also give others a better starting point to help you. Taking a photo of the condition of the area of concern will also help.
post edited by 2lateforPplate - 2021/05/01 15:07:44