I was told once that all vehicles, including ambo, trigger it. Don't know if it is, or even was ever true.
According to CheatCommands mod, nope, in fact, the list of vehicles that trigger it are a bit strange. Using the mod to spawn units near a vet1 Sherman M4A3, we get the following units triggering the effect:
OKW:
Panzer4
Panther
Sturmtiger
KingTiger
JagdTiger
Hetzer
Command Panther
JagdPanzer 4
USF:
M10
M36
Easy Eight
Bulldozer Sherman
Pershing
Calliope
UKF:
Comet
Cromwell
Centaur
Churchill
Churchill AVRE
Churchill Crocodile
Valentine
Firefly
SOV:
IS-2
T-34
T-34/86
T-70
M4C Sherman
KV-1
KV-8
OST:
Panzer4
Panther
Tiger
Tiger Ace
P4 Command Tank
Elefant
After a bit more testing, the other vehicles that gain Radio Net (Easy Eight, Bulldozer) also seem to follow the same criteria, so I assume it is drawn from a single list that applies to all units with the Radio Net mechanic.
Looking through the results, the list isn't very consistent:
For light tanks the T-70 and Valentine counts, but not Stuart or Luchs.
Some turretless vehicles like Axis StuG III and JadgTiger triggers effect, antitank units like Elefant and M36 as well, yet the common Allied SU-85 doesn't.
Heavy assault vehicles like AVRE and Sturmtiger, but not the Brummbar.
Calliope counts but all other artillery vehicles like Priest does not.
As you can see it's all over the place. Some units that logically (and illogically) trigger the effect you would expect like most Allied medium armour, yet a small number of vehicles that fit those categories don't. I can understand Brummbar, but not Stuart? It is not like the Stuart itself would benefit from the effect, and the effect isn't even that great to justify spending resources just to have a light tank sitting nearby, and you'd think it would promote multiple unit gameplay tactics.