I bought a new one because I couldn't get the old one open, and didn't want to press the issue if the car might be undriveable as a result. After installing the new, I tried harder on the old cover, and it did finally come loose. A small amount of contact cement was responsible. Although I cleaned the relay contacts and resoldered the terminals, I keep the old one as a spare - I haven't tried it again.
Check on FastLane or other internet sources - they are much cheaper than the dealer.
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7