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I am just about to start installing a new headlining. I have removed all the plastic fittings for the sun visors and turned my attention to the interior mirror.
It was only fastened in with 2 screws, (the centre one was not there) however I have snapped off both screws! Any ideas on how to remove/replace these screws? I believe they are self tappers so will be very hard and almost impossible to drill out.

They should be oval head machine screws ... just rusted in.

I would try and drill them out personally... if it all goes pair shaped, put rivnuts in

They are not self tappers they are machine screws and soft enough to drill out. Someone will know the correct thread, probably 3/16 UNF.

Gaffa tape some copper washers in and weld a rod (really with a t handle) onto the end of the thread very carefully. The heat helps to dislodge them and the copper stops the weld going onto body.

My son has a real knack of doing this!

I would also go the welding route.

never had much luck with left side drills or similar...

The correct fittings are 10/32 x 1/2" UNF countersunk machine screws.

I would drill the broken fittings out. I use 1/8" drill and carefully drill a pilot hole as close to centre as possible. Sometimes you'll get lucky and the drill will catch in the broken fitting and unscrew it right the way through the screw hole. If not I go up in drill size as close to hole size taking care not to drill out the original thread, then clean the threads up with a tap of the appropriate size.

Had a go today at getting over this problem. I drilled out the bolts, I tapped the first one M4, I know it should be imperial but as it was only the interior mirror it would not matter too much.
First tapping went fine, the second one was a bit stiff whilst tapping I wound the tap back to clear the threads, thinking to myself I hope the tap does not snap, guess what, the ‘kin tap snapped!!
I have tried to get the broken bit out with mole grips, but the broken part of the tap just kept snapping until the was nothing left to grip. I don’t want to use any heat as the car is painted and I don’t want the paint burnt.
I think I may just relocate this hole slightly to one side and re tap the hole.

I don’t think you’ll be able to relocate the holes - they will be captive nuts so moving them means you’ll just hit sheet metal. You could either:
1) Just use 2 fixings and hope they are sufficient
2) Relocate but use countersunk self tappers

GraemeC. I only intend to move one hole a couple of mm one way just so it misses the snapped tap.

I would just go with the two fittings and leave out the one with the broken tap. Maybe cut the head off the last screw and glue it in place so it at least looks right. Will make a nice surprise for the next person that pulls the car apart in who know how many years time.

Minibitz, the is a plastic cover over the screw so they won’t be seen.

Use the 2 good holes that you have & cut a screw head off & glue it over the offending balls up.

The 2BA screw that was there in the first place was closer to M5 than M4.

Should be fine, its not like it takes much weight is it.

It might vibrate with only two screws

Modern cars use bonding agent or tape - I imagine that would be OK with the two screws clamping in in - unseen of course?

Just had a hydro valve snap on me and used an extractor in the hole, just like your tap, that snapped the moment I put any pressure on it, ggrrrr. Luckily for me if needs be I can remove the whole union.