Is there a simple way to make the numbers illuminated? I'm happy to mod your mod myself locally if I need to.

The bearing was always available regardless of light conditions, and was completely different from how it is depicted in game. For one thing, you could just look straight up to directly view the ring(s) at the top of the scope[1]. Even better, both scopes had a prism system[2] that simply looked straight up at these rings and reflected that image to appear at the top of the view inside the scope[3]. In other words, yes, only the crosshairs and not the angular markings were illuminated in the graticle -- however, the bearing indicator was not part of the graticle!

I'm tired of being unable to get something so essential and definitely historically available on both scopes. Definitely both the C/6 and C/9 had the prisms, and looking up was always an option, and obviously you'd have a decent sense certainly to within 30 degree of your orientation in the boat, and attack scopes had a 4-digit mechanical counter[4] showing true bearing to the tenth of a degree, but none of these are available in game other than the bearings at the top of the scope view. The illumination to read the bearings is just the interior illumination of the sub; unless you had total electrical failure you'd be able to see the bearing marks just as easily and using the same illumination as if you just looked upward.

1: For example, http://tvre.org/images/16_fot_12.jpg on U-995. I tried just using freecam to look up but sadly these are nonfunctional in game - they rotate with the scopes. On the observation scope, 0 moves with the eyepiece, instead of staying fixed to the hull with 0 always indicating the direction of the bow like they should be. On the attack scope it's 295 always.
2: That's what https://i.imgur.com/YVAlJ14.png this box is, just a prism with a lens looking straight up at the ring. This is the one on the back of the observation/night scope, which had two azimuth rings for this reason (and this reason only) - the second was offset 180 so that the view through the prism would show the correct bearing. A diagram of the layout of the c/6 night scope is available https://i.imgur.com/4P6oeKr.png with my annotation, where these parts are clearly seen.
3: Shown at http://tvre.org/images/16_rys_18.jpg for the attack scope, which had a single azimuth ring plus the colored deflection ring, because its prism was on the front.
4: This is in the game, but nonfunctional; it always displays 1500 regardless of where the attack scope is pointed.