Well if you get it when braking and all of the bulbs are matched as they should be and they light then I would suggest that you need to replace the actual metal terminal connectors at the ends of the wires. We did so many of these back in 89-90 that I actually had all of the terminal positions,wire sizes, and terminal sizes memorized. What does that tell ya? We did a friggin' boat load of those darn connectors. They changed up to a beefier style of connector, it is rather tough to tell unless you know what you are looking at and where to look but the old school stuff had essentially 4 little lips (two on either side) that made contact whereas the new and improve one looked the same (2&2) but behind the lips it had another shorter set of lips to reinforce it and keep the tension on the connection up. Should you decide to change these out you will need to get the correct connectors from the Volvo parts department and you will need the correct style of crimpers to do it with. They cannot be done with regular crimpers so do not waste your time and money trying to do so.

Mark