I am just restoring the bumper assemblies on the last of my 3 cars! The small Philips/posi drive screws that hold the shiny bits onto the plastic bumper are a problem to remove as anyone will know who has delved in there! I had to drill most of the heads off and they are a bit "hard" but with a tipped drill bit it is possible.
Now to identify the thread to replace them with stainless screws. I think they are M3.5 which has a 0.6mm pitch. Can anyone confirm this? My thread gauge doesn't include one for 0.6 mm! In the UK M3.5 is the thread used for the screws which hold light switches and sockets to the back box. These in the old days were chrome plated brass but now tend to be EZP steel. I have some repair techniques and removal techniques for bumpers that have seen road salt on the mountings. I couldn't see a post on this so when I get a minute I will write something up with pictures.
KR
Charles