80 Estate aftermarket stereo problem

[Cabe]

After extensive trawling I am unable to find a solution, hopefully you fine bods can point me in the right direction.

I have just bought a 2.0l 80 Estate and after ripping out the old (after market, I think) Sony cassette deck and putting it into my old car, where the old HU worked perfectly. Sadly with the reverse, No Dice.

It doesn't seem to be providing enough power (always on only, at some point I will steal me some ACC power but that's not important ATM), it just flashes on and off once a second and makes a pitiful clicking sound if you eject the disk. Getting the HU on the bench with a decent 12v supply solves the problem. I am getting 12v out of the car, irrespective of whether its running or not.

Its a Sony MEX-BT2500 MP3 CD Bluetooth unit if it makes any difference.

Help please, I'm losing my mind without my music!

Cheers