The add-on feature was done by the delears, maybe it can be disconnected and see if that is what is discharging your battery.

DEWFPO
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1998 S90 077,400 and 1995 964 154,100

Just because 3 batteries and 3 alternators have been replaced, does not mean they were all needed. It is more likely that even if the first battery was bad, that the next two were not anything more than discharged. Your complaint of intermittently dead battery may be your whole problem.

Many things can make an alternator appear to be bad. Yours may have been replaced on the assumption that it was responsible for dead batteries. Were the replaced units checked on-car and then off-car?

I had a similar problem with one of my own cars. Sometimes parked for weeks and fully charged, other times could be almost dead sitting overnight. First, I found a stuck power seat relay, and then finally found that the aftermarket alarm would sometimes pulse the parking light relay every 20 seconds so briefly that the outside lights didn't even have time to glow. The constant pulsing drained the battery in hours. This gremlin sometime didn't rear its ugly head for weeks at a time, making it very difficult to trace.