They are using the ones that look like a fog light or headlamp relay on the later models, right? The ones on my old '88 look like the old school Ford fender mounted starter solenoids. And they are still the original ones. I'd make sure that you are getting ones that are designed for damp environments. I used a Painless Wiring relay harness in my old '98 Jeep to improve the headlight wiring and it came with Hella relays in a waterproof holder. I have never had to replace them either and they've been in a salt air environment for close to 15 years.
The trim system draws a lot of amps, second only to the starter motor so anything that increases resistance in the wiring will cause trouble. Make sure your batt cables are clean, tight (including the engine ground) and check all the connections in the system for any corrosion. As I said in the other post sometimes you can get a break in the trim switch wiring on the remote control handle (inside actually) that can cause intermittent problems.
Bosch or Hella make good relays, as long as they are still made in Germany or USA. Anywhere else don't buy them.
Depending on the model #, there was a combination relay pack or this style where they were separate plug in ones. Is this what you have (this is the OMC version of the joint venture model, but should be the same as your Volvo)....relay is part # 25
http://www.crowleymarine.com/johnson-ev ... /56995.cfm