It would fail an mot of course but yeh, easy with a couple of diodes (min 10A rating). Hardest bit is hacking into the loom and splicing the wires without making a mess. The diodes will dissipate around 5w so would need to be on a small piece of aluminium heatsink or similar. Search 10A stud diode on google to get an idea. It's critical that the diode and any associated heatsink doesn't touch chassis of course as that is ground!