2003 Silverado Amp Install?

You are fine getting the signal off of the rear doors as long as the LOC you have can handle that "hot" of a signal. If the amp has a crossover on those lines then it will be necessary to go to the front door speakers. I really doubt that there is a crossover on the rear speakers. PAC and others make some very good adaptors that will work just fine for you. Another option is a good high power LOC ----> active crossover with dash mounted level control ----> amplifier. This will afford you the flexibility of controlling the amount of sub bass indipendent from the level of the rest of the speakers and will allow you to leave the potentially damaging bass control on the stock deck set at minimum. Have a look at a nice little gadget like the Crossfire CX2L + bass remote (if it does not come with it). It has a good LOC built in and will handle most everything that you need it to do.

www.crossfirecaraudio.com