This is fairly simple, but I still found this clever of Rock star to stick this into the game.

When you go to Zero's RC store, look behind the counter at the magazine racks, there will be Tommy Vercetti and Lance Vance action figures for sale(Though you can't buy them). On some other racks in this store, next to the counter, there will be Cash and Pigsy dolls from Manhunt!

Also, when you go to a bar(Any bar), there will be a SWAT officer from Manhunt painted on the wall.