Indicator fault - opposite side flashes dimly

[eltonbird]

My Mk2 Turbo has just developed this fault, where the opposite indicator to the one activated flashes (both the actual light and the dash signal).
It flashes at about half the rate of the desired indicator, and not at full birghtness. Hazard on works as normal.

Anyone have any tips where I might look to diagnose? This seems to have started at the same time that my immobilser has stopped working!

Any help appreciated!