Best way to remove the fasteners holding the rear valance to the rear wings?

[mrtask]

The rear valance is attached to the lower corners of the rear wings with a pair of short chunky screws with a big wide flat head. The two grooves for a screwdriver or similar are very shallow, and I can't get purchase on the side of the big wide domed screw head with mole grips or similar as they are sunk into a recessed dimple in the valance. Anybody got a good idea what tool to use to unscrew these blighters? I haven't got a screwdriver with a wide enough tip to be able to budge 'em! What would be the correct tool? Is this a hammer and small chisel job?

 

[cobraboy]

Soak behind with 50 - 50 mix of ATF and Acetone.
Deepen the slots with a wheel on a Dremel then try one of these ?
Yamoto 10mm, Metric, Screwdriver Bit Socket, 1/2in. Drive 5827810K | Cromwell Tools

 

[harveyp6]

ISTR they screw into a plastic insert, so shouldn't be seized, and as standard they were large self-tappers with a bolt hex head.

 

[mrtask]

Thanks Mark, great tool tip. I didn't know little stubby wide screwdriver bit sockets were a thing.
Harvey, on my car they are screwed into plastic inserts, yet one of the bloody things is stubbornly refusing to budge. Despite liberal application of penetrating fluid. I did wonder what twisted psychopath specified these fasteners at a company renowned for their engineering expertise! A gift from a previous owner, presumably.