Accessory Signal in the Fuse Box?

[guruerror]

After much frustration with several aftermarket headunit wiring harness interface kits (PAC, Peripheral, Metra, etc.) I decided to completely bypass these units as I don't need the door chimes and have already removed OnStar (to make room for the CB). I've wired straight from my headunit to the factory harness (directly into the Bose amp).

SIDE NOTE: this works great...my volume levels are higher then when using any of these kits...the tech at Metra kept saying that it couldn't be done..."it's a digital Bose amplifier, it won't work"...whatever, it works fine.

...except for one thing; I obviously have lost Retained Accessory Power. Not a big thing but can someone point out a good fuse in the cab fuse box to turn my headunit on when the key is switched to Accessory instead of all the way to Ignition?

I'm currently tapped into an 'unused' HVAC fuse to get an ignition signal; with this setup my headunit turns on when the engine is fired up and off once the engine is stopped, but does not stay on when the key is left in 'Accessory' position.

NOTE: I'm not searching for a 'power source'...my headunit taps straight off the battery...I simply need a 12v signal from the fuse box for Accessory or Retained Accessory instead of Ignition.

Can someone point out a good fuse to jumper off of to get this signal?

I've seen some posted regarding the use of a relay...if this is my only option, can someone give me a little more detail regarding what type of relay I should purchase and how best to wire it up?

Thanks in advance.