Highlighting over flat black paint

Add some red-brown to your black and it will give a nice faded black look without it being too stark. Dry brushing with a dark gray or neutral gray should look good too. Deck Tan or Buff added to the black would work too.

If you have molded details you want to pop, mix some grey oil washes to add some depth. When dry, then  dry brush as Nathan mentioned. I found that a light wash of yellow ochre oil paint will fade a bit and separate the molded detail against the black.

I seldom use flat black for actual surfaces. I only use it for places that are an orifice where nothing should be visible.  For all else, I soften the black. I find panzergray is a good very dark gray to either use as a weathered black or to mix with black to soften it.

I second that on Panzer grey. Actually it's called Panzer Schwarzgrau RAL 7021.

I second Don and 214.

However, if you're not happy with highlighting with grey, perhaps a dark blue might create a highlight without contrasting too much with the black.

One possibility for wheel wells is a very feint, light wash in just a few areas of red, to simulate hydraulic fluid. I sometimes do a very light wash of this on the gear too.  It has to be absolutely minimal, however.