This particular diagnostics cartridge does not require a test harness, and are specifically useful when your C64/128 has no picture. What sets this cartridge apart from the other test cartridges, are that they all require the RAM to be functioning to boot, (unfortunately, one of the most common fault is defective RAM) This cartridge not only work with defective RAM, but it also can tell you which RAM is broken. The reason why this is possible, is that a correctly made DEAD TEST CART will not require the system RAM, but it will instead temporarily overwrite the kernal ROM bypassing a lot of functions needed to boot the computer. A random person can obviously just program his all purpose C64 cartridge PCB with the ROM file from the DEAD TEST Cart, but it will be useless when you really need it. Thats why I made this PCB specially for this one purpose, and did not just throw a ROM into a all purpose PCB.
Here is a link to the user manual.
This project with PCB and parts can be purchased on ebay: https://www.ebay.com/usr/kirsti_73
or from Sellmyretro.com