I added a subwoofer while fixing up my cab and had enough parts left over for an extra loom.
Looking for $45 shipped in the USA. May seem kinda pricey, so here was my cost breakdown:
$16 Hi-Lo converter
$6 Monster RCA 6ft. cable
$4 RCA plug
$5 connectors, wire, ties, ect...
I basically just followed the diagram halfway down this link. I cleaned up the design a bit so no alterations have to be made to the original loom...
https://segamadebaddecisions.wordpress.com/2012/06/18/my-astro-city-stereo-mod/

Any sub with RCA inputs and outputs should work just fine. This is the one I used:
https://www.amazon.com/Logitech-Watts-Multimedia-Speakers-980-001203/dp/B0151K2AB0
Also, here's some drop in replacements for the factory speakers at the top of the cab too:
https://www.amazon.com/FaitalPRO-Neodymium-Professional-Full-Range-Woofer/dp/B00F0XNU5E
Looking for $45 shipped in the USA. May seem kinda pricey, so here was my cost breakdown:
$16 Hi-Lo converter
$6 Monster RCA 6ft. cable
$4 RCA plug
$5 connectors, wire, ties, ect...
I basically just followed the diagram halfway down this link. I cleaned up the design a bit so no alterations have to be made to the original loom...
https://segamadebaddecisions.wordpress.com/2012/06/18/my-astro-city-stereo-mod/

Any sub with RCA inputs and outputs should work just fine. This is the one I used:
https://www.amazon.com/Logitech-Watts-Multimedia-Speakers-980-001203/dp/B0151K2AB0
Also, here's some drop in replacements for the factory speakers at the top of the cab too:
https://www.amazon.com/FaitalPRO-Neodymium-Professional-Full-Range-Woofer/dp/B00F0XNU5E