Checked the wires for shorts, all fine. One of the rear pair of speakers is 3.7 Ohms, the other 0 Ohms so definitely duff.
I've had loads of old bangers with a duff speaker, I'm just so surprised that one channel can cut the entire amplifier. Learn something everyday.
On the plus side the new head unit has bluetooth and in-built mic so the TR instantly has mobile phone connectivity.
Can anyone use a Blaupunkt Atlanta radio/CD player that probably has nothing wrong with it?
Jerry