IF your '95 D90 has the Zf 4HP22 auto trany in it, as does my '97, here's the deal, in this transmission, the flow through the lines is one direction for gears 1 through 3, and in 4th (which is over-drive & torque lock-up) the flow reverses through the cooler (which, to be sure, is located below the radiator). With the flow reversal, you would NEVER want to install an in-line filter on this trany.
Also flushing a Zf 4HP22 is not always a good idea UNLESS the trany has had the fluid changed regularly (60K miles). What will happen on un-maintained tranny's is the new fluid will cause the liberation of tiny particles from the clutches and cause plugging of the valve body. These particles go right through the screen filter in the pan. It's best to change out 1 quart per year in this case. Bad fluid will smell burnt.
Should you decide to put an in line temp guage sensor on it, you'd want it on the fluid out line for gears 1-3 (4th gear only kicks in at about 52 mph). This is the line coming out of the trany at the front bottom. Bad thing about the cooler location is that at low crawling speed (where elevated trany temps are most likely to occur, unless your towing) there is no "forced air cooling" as there is in the radiator with it's fan.... bummer huh?
The Zf 4HP22 is a "most excellent" trany. My '85 BMW 524td (same gear ratios as the D90) has one and is still performing wonderfully after 26 years with only fluid and filter screen changes