1987 Defender 90 - Weird electrics?

I've done a few searches and realise that electrics are a common problem on Defenders, but I still cannot see the best way of fault finding my problem. Basically, If I just have the radio on, all is ok, if I then switch on the sidelights, when I indicate the radio cuts out, glitching in time with the indicator flashes! If I turn on the headlights the radio sometimes cuts out completely, depending on whether the heating fan is on slow or fast. It all points to a lack of current somewhere, the landy starts ok every time so the battery appears good. Where do I start looking?

Thanks guys.

Ppoor Earths. Clean all the contacts on the lights and bulbholders. run seperate earth and live feed to radio.

Also worth cleaning all the brass ends/or steel ends on the glass fuses, they age and the contact is sometimes lost altogether.

Thanks guys, I'll get busy at the weekend.

Cheers

Could this also be why my engine cuts out when and indicator is used, and sometimes for no apparent reason at all (solenoid checked, fuel system checked all OK)?