I feel like blockade battlebuses are already strong enough versus infantry without also knowing where all infantry players within 50meters(?) are. Plus the radar only works when someone's in the sundy so it wouldn't be all that much help to deployed sunderers. If you're going to gun an AMS, might as well put a motion spotter down next to it anyways.
I rarely fly but I certainly understand why some people were opposed to giving aircraft scout radar as default. I feel giving default proximity radar to any ground vehicle just encourages even less situational awareness. I wouldn't view it as a tool but more of a crutch. If you aren't always playing infantry with your head on a swivel and using 3rd camera free look often in vehicles you deserve to be a statistic.
I would say turn proximity radar into scout radar in order to be a recon bus, maybe extend the range much more. No ground vehicles have scout except flashes.
My apologies for being direct but you are wrong. The ANT also has Scout Radar as a utility slot item. I am currently loving the Scout Radar on my ANT for certain situations. For the Sunderer however I would go with other defensive slot options. Then deploy sensor beacons or the humble beeper gun for an additional heads up as others have mentioned.
sunderers are already powerful as they are, a better idea might be to have it only work as an innate ability when stealth, shield and blockade aren't equipped, because if it did work with those it would be OP against infantry.