No sound - installing new Alpine HU in 1999 Denali w/ Bose?

unfortunately you have a big expensive problem. the factory bose systems are costly to change the head units on because you have to install all new speakers also. i've had to deal with a few of em. the first one i had the same problem as you and found out each speaker has its own small amplifier built into it. to change head units you gotta change the speakers too.