Sorry about the foul-up on my previous post.
The fuel filter on your '90 300SE is in the rear just to the left of your right rear tire. There's a plastic shroud that covers it and the fuel pump. It's accessed more easily if you back the car up on ramps. Both ends have a large nut attached. Slide appropriate
size open end on these nuts, then loosen smaller nut. Some gas will spill out. As for priming, I've never had a problem here. You will have to crank the engine a bit until the fuel get through the system. Also, be sure to pull the ground strap off the battery B4 you begin work.