Head Unit switch off with speakers connected

[Willfin]

Ive recently had my TVR laid up over the winter, and removed BECKER head unit as i didn't want it sitting in the car.

Anyway, Ive gone back to fitting it, and when the power plug is connected the unit powers on fine, but when i then connect the speakers plug, it turns off!

Any reason why the unit would turn off when the speakers are connected? Back earth somewhere?

Thanks in advance.