Preferably in English and not for the 817ND!
Every link I've ever been given is response to this question elsewhere actually refers to the 817ND and no, they aren't the same.
I finally found a FT-818ND specific resource.
This page includes a MARS/CAP TX-RX modification for the FT-818ND :
https://www.mods.dk/index.php?ModelId=1274&ModelRec=ft-818&RadioId=1-yaesu
According to the article, the modification is done through SMD jumpers/diodes on the main board, and the exact procedure may depend on the regional version of the radio.
Be careful though: some FT-817 information partially applies to the 818, but not all board layouts are identical. A backup of current settings and a full reset after the mod are usually recommended.
Thanks for digging up that link Davys, that's super helpful.
You're completely spot on about the board layout differences. The FT-818ND main board has minor revisions around the microprocessor area compared to the older FT-817ND, especially regarding the jumper layout. A friend of ours tried using an old 817 diagram on an 818 a while back and almost bridge-soldered the wrong SMD pads.
One quick detail for anyone planning to attempt this mod: make sure you save your alignment settings or write down the soft-cal menu values before doing a full master reset. Sometimes opening up the wideband transmit can reset factory calibration values on certain firmware revisions.
Thanks again for sharing the exact link for the 818.