I'm assuming that your problem is not how to key from the back bus (which you can do by simply routing the key signal to the back bus, but not using the back bus output of the compressor / main fader).
I'm assuming your problem is how to stop the front bus also keying the compressor while you are keying from the back bus? Ok, the way I would do this (I think) is to cut the link between pins 17 of the '26 cards on the 651 backplane between the front and rear cards. So the front cards are still linked, and the rear cards are still linked, but the front and rear are seperate. Now you'll need to find another switch! The existing compressor In/Out switch will switch in either the front or rear compressor key input (I don't know which, it depends which pair end up connected to the switch after you've cut the track!), then you can wire a new switch to the other two pins 17 with the switch pulling up to +18V and the lamp pulling down to -18V. (The pull down to -18V isn't essential, R28 on the 26 card should do it anyway.) This should make sense if you have a 26 card circuit.
Now, assuming your main signal is going through the front bus, your signal can be keyed by either the front bus, the rear bus, or both depending which of the switches you press (or both together!). Or off if both switches are off.
Hope that make sense, it's hard explaining in text!
Post here if you get stuck
By the way, this will invalidate your SSL warranty
Cheers,
Andy
