That circuit can fail for a multitude of reasons, the usual culprits are (in no certain order) the brake pedal switch, the ignition contact plate, the micro switch, or the solenoid itself. Back in the day we replace a boat load of micro switches, especially in 850's but we did plenty in the late model 900 series cars too. Which, by the way, are the biggest pain in the ass to do. While I still do the rare micro switch these days, I find myself doing far more ignition contact plates. My advise is to diagnose it properly and fix right the first time. The contact plates darn near diagnose themselves, when the problem duplicates just wiggle the key back towards you a little bit and see if it cures itself. Another dead giveaway of a bad contact plate is that there will usually be other things that do not work either like the radio or the lights. The 960 that I am currently driving does it like clock work every stinking time I start the car. You'd think a Volvo wrench would fix something like that but it is liken unto the cobbler's kids and their shoes, know what I mean?

mark