You can get the harness side connector from places like Rovers North. They sell them as repair parts for the rear lights on 95-97 90s. They come with the connector and a few inches of wire that you can splice in (or put Lucar connectors on to plug into your 94 original wiring) and that will allow you to plug it into the lamp. That or you can source just the connector itslef from the UK from Durite.
Be aware though that unless you drill new holes and buy a new plastic headlight trim to mount the signal lower, like a 95-97 front signal, a 95 style larger signal is going to look real goofy mounted higher in the 94 D90 location. (or at least I think it would look out of place).
I do not check PMs. Call or email if you need something.
ECR is not a parts source. We are not in the mail order business.