I've dealt with a few of those aggravating bose systems in the past and they can be a pain. Upon reading the post, my first thought was to tell you to check http://www.the
12volt.com, but I just did and they don't have anything listed for your truck. Here's an old school alternative: If you can locate the amplifier and get a good look at all wires providing input/output to the doors, rear, sub, then that could possibly mean that you could find out which is for the sub through a process of elimination. Remove the sub from the enclosure and see what color wires are attached to it and there you go!!! Often times factory wire colors remain the same throughout the system. I've had to pull door panels and rear deck to see which worked for each speaker in cars where I had to bypass the bose factory amp to run aftermarket equipment and didn't have a wiring diagram. However, I've always been told that an amplified signal is the worst for input for a LOC because it can introduce cray noise, fedback, etc. I was taught to find the input section of the factory amp and take my inputs from there. Hope this helps, man. Take it easy!
1998 BMW 540i
Headunit: Stock DSP; Hifonics MX-1
Front Stage: Cerwin Vega 5.25's
Rear Stage: OEM 5.25's
Sub: RE Audio SX 10.4
Amps: Rockford 400.4 & bd1000