@Von Bocchini✓ˢᵉⁿᵖᵃᶦ It's because the mod developer made it for SP. It doesn't work without editing the config file to use MP instead of SP. You have to change the line in fnpostinit with in functions from:

"klpq_musicRadio_enableBackpackRadioSP",
"CHECKBOX",
"All Backpacks Can Play Radio (SP Only)",

to

"klpq_musicRadio_enableBackpackRadioMP",
"CHECKBOX",
"All Backpacks Can Play Radio (MP Only)",

and it will work in multiplayer

KLPQ music has to be active in the addon options menu for it to work

I'm afraid i don't know, maybe because this haven't been updated in a while and i lost the files for this mod

KLPQ music radio mod doesn't work in multiplayer, do you know why?

Also you can do it with Zeus

more precisely it's in ace menu in the car and if i remember correctly backpack LR radios

how to use