Which antenna cable for aftermarket HU
 

I am Installing a aftermarket headunit receiver in my FD. The previous owner already had a aftermarket HU in the car when I purchased it, but I am looking to upgrade.

I noticed one of the antenna cables was cut and spiced into the HU. The antenna cable that was used was the smaller diameter cable. I read somewhere that the larger diameter cable is the correct one to be used for a aftermarket HU. Does the larger diameter cable connect to the antenna that actually raises up from the car? Anyone have knowledge about this and can confirm this?

I do believe you're right. The smaller one goes to the antenna that's built into the rear hatch glass.

They might have wired it like this because the power antenna didn't work or they couldn't figure out how to wire up the antenna.

Dale