I tried installing a subwoofer from my old fiesta into my new one, but now find the headunit has no power

- the sub was earthed, connected to the 12v, the remote was attached to the headunit, via the wiring harness from the old cd player . nothing worked

- then i tried using the wiring harness in the new car and patched the remote in. nothing happned

- i then tried disconnecting the sub, and put everything back the way it was, and now the headunit wont turn on. pretty sure its not the headunit, as i tried the old cd player as well, and no power comes on to either of them

- fuses in power headunits are fine

anyone got any suggestions, as i am at a loss of what to do next and i have a 4 hour drive on friday night and dont want to do it in silence !