I suspect rivinuts won't work, the threaded part is more than likley a stud on the base of the mirror....
if you can get a finger tip onto the end of the thread when your are trying to mount then it will be easier, the best way tends to be using some ratchet spanners.... they come either as fixed head or as flexi hread... if you can borrow a set all the better as they ain't cheap... the Draper Expert ones are what you really need as they are slim and have a reasonable ratchet action... next pop the nut into the ratchet end and using a bit of glue (you can try butack) I tend to use superglue as it isn't that super in reality and then you can use the ratchet action to tighten the nut...the glue can simply be detached by a gentle rocking action.
if you have an assistant to hold the door/mirror all the better but on your own it may be poossibe with the window open..
if you can't reach the nut then it's a case of getting the nut onto the end of the thread.. this is where the flexi heads can be useful, sometimes wrapping aking tap ove the flexi part to sop it slopping about when you have established the angle to line up the threads..
Tip -if you buy a set don't lend the out (despite me telling you to borrow one) as they are not really designed for being used to tighten things to breaking point as they will come back knackered... as I can testify to.