How come my 12 inch subwoofer wont work?

so we routed all the wires we need from the battery to the amp to the sub, etc. spliced the audio from the amp to radio into the rear speakers in my car and we spliced the power to the amp from the radio with the correct wire there that is hot while acc is on, and we had hooked up the subwoofer to L+ and R- for the mono sub and after we tested it out, we got a repetive constant beat (motorboat effect [which can be caused by a wire to the sub from the amp being grounded somewhere else maybe, but we checked and nothing is being grounded accidentally..]) that went even when the radio was off (with accessories on). but when we had put them both on L+ & - or R+ & -, it would work fine, but with the bass on the amp all the way up and the bass on the radio all the way up, it just moved without making any vibration, but it was following the beat for sure. Is there something we missed?