I bought connector 15 years ago for old volvo 99'
It was $7 in walmart.
It still present in 2015 truck. Just after Metra connector.
But probably rear speakers in bunk will not work. But is it really needed?
If you buying aftermarket radio make sure it have build in crossover to limit low frequency on dash speakers and high on door speakers.
Anyways if it's your truck - replace speakers, you will never get same quality as stock radio. It's long story why.
Buy only component speakers, not coaxial. And any amplifier, even cheap.
About steering wheel buttons. 99% it will not work. It work via CAN-bus. It like computer network in modern vehicles.
Some adaptors probably exists....