Those are early "SmartCraft" gauges and need a CAN (Control Area Network)
harness and junction box. This link shows a newer version of SmartCraft
gauges, junction box and harness.
SmartCraft harnessI think I may have a Mercury Technician Guide for the early SmartCraft system
around here somewhere.
Older SC gauges have the "eyebrows" just above the rubber pushbuttons
under the lens.
ETA -- Here are some pics of a Verado SC install I did for the Sheriff's office
Lake Patrol. They opted to keep the analog gauges hence the Analog Gauge Interface box.
The CAN junction box is to the left of the AGI.
Overlook the rats nest of wiring the Sherriff's guys put in there above my SC/DTS install.
There are different versions of SC systems so you have to make sure you
get the correct cabling or it won't work.
I'll hunt around for that guide, also try looking for a SC rigging guide for the older SC stuff.