Perhaps you could flip the key on and use a tester to find a hot circuit and tap the buss at that location? use velcro and stick the relay to the side of the fuse box?
I suck at using the search function but this issue has been discussed in detail.
Sorry if that doesn't help, or i am not on your page. There are guys here that can tell you the fuse number that'll work best... I can't/don't memmer.