You can code is as an extension building, but then you need to specify the waytype.
Obj=building
Name=Garage
type=stop
extension_building=1
enables_pax=1
enables_post=1
Level=9
intro_year = 1977
intro_month = 1
NoInfo=1
BackImage[0][0][0][0][0][0]=garage.0.0
icon=> garage.0.1
cursor=garage.0.2
However, I recommend you to code is as a commercial building, using this dat file..
Obj=building
Name=Parking
Type=com
copyright=Propermike/rfg123
Level=2
chance=100
intro_year = 1977
intro_month = 1
climates=desert,tropic,mediterran,temperate,tundra,rocky,arctic
BackImage[0][0][0][0][0][0]=garage.0.0
Imho, it fits better at that category. Anyway, when you're using Pakhelper and you suspect something went wrong with the pak file, it's good to check the log file called pak.log that contains the makeobj output messages written during the compilation process. If there's something strange or wrong, It should appear there as a warning message.