Have an old late 70’s/early 80’s car stereo/am/fm/cb radio. It nase 4 wires coming off of it. Red, black, grey, brown. The black wire is bolted to a stud on the chassis of the radio. I powered the radio up using the red + and black for - . I am assuming the grey and brown wires are right and left speaker wires. question is, when wiring speakers, should the speaker neg just be grounded to a meta part of the car, or ran back to the black wire on the radio?