That sounds like the power steering resevoir. Should look about the size of a small coffee can. Unscrew the knob and lift off the top. There is a line stamped in the body, about 1/2 inch below the rim, that's the fill line, the filter should be under the plate with holes in it at the bottom. Any quality power steering fluid will work.

__________________
Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.