We use Kannad 406 unit (on aircraft with the 121 units fitted previously, no problmes we just "plugged" unit into existing system.
We bought some aircraft from the US last year and the fun started as no ELTs fitted. Switch in cockpit, ran loom down cabin ceiling and ELT406 in aft of baggage area with "shark fin" antenna punched through the roof. Had to get mod and structural drawings approved.
Ensure your cockpit ELT switch cannot be hit by pilots, Jepp manuals etc (or you will be getting calls from the rescue agencies.
Type KANNAD 406 into google for all info