Combo 2013 - wont start but what is this plug???

[Jimmals]

So I had the starter motor replaced on my 2013 Vauxhall Combo Van (Fiat Doblo style) at a garage, 10 miles later it stops. Nothing when key turned, dim dash lights when key removed, but.....

This plug is hanging down the back, it clearly belongs somewhere but can I find where? God no. The wiring from plug goes back and splits, with one end coming to sit on top of the engine.

Please help.