2000 PF with Bose system front speakers popping

[Andrew101]

Hi, I recently bought 2000 Pathfinder with Bose system and noticed, that front speakers doesn't work. They just sometimes randomly pop for a second and that's it.

After doing some research, I took off rear amplifier and checked if all the wiring are not loose. They all were in place.
Then I thought, that the problem may be in the factory head unit, because lights on it sometimes didn't work, sometimes work.. So I ordered the scosche wiring interface from crutchfield and installed aftermarket stereo. Rear speakers works great, but I can still hear popping sound from the front ones.

Then I've found the manual and that's what it says:

All front speakers are inoperative
1. Audio amp. relay - Check audio amp. relay.
2. Audio amp. relay ground - Check audio amp. relay ground (Terminal 3)
3. Amp. ON signal - Turn ignition switch ACC and audio unit ON. Verify battery positive voltage is present at terminal 1 of SC audio amp. relay.

Could someone explain how to check audio amp. relay ground and what is the terminal 3?
And how to check battery positive voltage presence? Do I need some tool for that?

Or maybe I can bypass factory amplifier somehow and connect front speakers directly to my aftermarket head unit? Is it possible?