radio turns off when braking

Hi there,  i have a 1989 cabby 1.8, when i brake for a long period of time the radio shuts off and ive just noticed the lights dim a little bit.

Jus wondering if someone could shed some light on what this could be or if anyone had experienced this before.

The car has had an after market stereo fitted to it which the previous owner wired it to the ignition so it only works when the cars started could this be effecting it ??

thanks in advance.

does it still do it if you arnt moving and you just stamp on the pedal?

It only seems to do it when I'm driving i tryed to do it last night when the car was stopped but all that happened was the lights dimmed a bit.

ok, suggests the wiring is a bit loose somewhere and moving about, in this case its best to take stereo out and have a good look!