The M4A1 Transformer was made on old non-steam Postal 2 version and sometimes it may not work properly.

If you have already a Jackal Crossbow mod, you do not need change the "L" key,
because it compatible with him.

If "L" or / and "H" don't work for you, you can try to do one (or more) these things:

1) Try exit game and run Postal 2 and M4A1 mod again.

2) Turn on / off manually - First open a cheat console (press " ~ ") and write "ToggleSpawnLight". Then, if you want turn the light on/off, you can open console again, press ArrowUp key (that instant write last cheat into the console), and press ENTER.

3) Add allias manually (lvl hacker)- First open the System folder in the SteamApps/common/Postal2Complete/ and find User.ini file. Then find these two lines:

Aliases[X]=(Command="",Alias="None")
Aliases[X+1]=(Command="",Alias="None")
and
L=ToggleInvHints
H=Dance

when "X" are the next "empty" allias (32 and 33 are for example), and change them to:

Aliases[32]=(Command="ToggleSpawnLight",Alias="M4Light")
Aliases[33]=(Command="CreateAccessoriesMenu",Alias="M4Menu")
and
L=M4Light
H=M4Menu

4) More simply, than ad.3) is run the game, and write in the cheat console these commands:
"set input L ToggleSpawnLight"
"set input H CreateAccessoriesMenu"
That thing automatically attach "L" and "H" keys with appropriate functions.