Electrical Fault 200tdi

Hi ive got an K reg 200tdi basically solid but there is evidence of some butchering to the electrics I was wondering if anybody could point me in the right direction for my problem; whenever I turn my lights or indicator on the power to the radio cuts out then comes back on also when I pulled out the fuse for the interior lights the radio cut out as well and it cuts out when I pull the proper radio fuse out.
an ideas ?
cheers

Did you solve your electrical fault?mine has developed the same fault!was thinking alternator fault(will check output tomorrow)or a bad earth to interior electrics?is there a central earthing point behind the dash?

Any advice before I rip the thing to bits would be greatly appreciated!

Thanks Josh