Please help radio install problems

[chevymotocross]

My truck came with an old Sanyo cassette player. The radio worked fine but obviously I wanted to upgrade to a cd player. The problem is, after removing the sanyo, I noticed the PO hard-wired the system. Anyways, I bought a JVC unit, matched all the wires, powered up but no sound at all from any speakers. I reconnected the old sanyo back to make sure it wasn't something I did, and the sanyo worked. So I returned the JVC thinking it was defective and bought a Pioneer unit. Wired up the pioneer, powers up, no sound! I notice there is a plug, coming from the truck's wiring end, looks like maybe an OEM radio plug, with 3 wires, black, yellow, and grey. Only the grey is connected, the yellow and black are cut and the plug goes to nothing. Why the heck am I not getting any sound from the new units but with the old Sanyo cassette!?