Installation:
Drop .ytd files in the designated location.
mods/update/x64/dlcpacks/spstallion4/dlc.rpf/x64/levels/spstallion4/vehiclesmods/stallion4_mods.rpf

Drop .gxt2 files in the designated location.
mods/update/x64/dlcpacks/spstallion4/dlc.rpf/x64/data/lang/americandlc.rpf or japanesedlc.rpf