If the rear o/s is not illuminated or flashing then the front won't be Peter. You need a certain electrical load collectively achieved from front and rear for the indicators to flash correctly, so measuring with what will be a high impedance voltmeter may not show 12 volts without a bulb at the front.
Suspicious are the wire colours in your pic from main chassis loom. To conform even by MMC standards the cables to offside indicators should be green/white and black. Your pic suggests two core mains cable has been used
Only thoughts are based on what I recall as your mention of Tarka being an MMC development car. Difficult to see but have the cables been extended and poor connection taped up where it exits from loom convoluted protection?
