trunk release not working
 

'00 sl500. r129, my trunk release makes noise like it's trying to open but it won't open unless i push the big round button on the trunk manually.

Anyone know what causes this?

Most likely you have a bad lock cylinder and the vacuum is not pulling the release open so you have to do it manually.

I had the opposite problem where my trunk would make noise but not lock. The trunk lock cylinder is not a common part and usually needs to be ordered from Germany. Have a mechanic check it out. If it is a leak in the vacuum system you need to get it fixed because it could cause other failures down the road.