The electric trunk latch has one feed wire for its actuator solenoid, and grounds through the metal-to-metal mounting of the latch plate to the vehicle sheetmetal. Run a wire from the trunk latch's feed wire up to the dashboard, via the left or right wire channel accessible by removing the left or right sill trim plates. Connect this wire to one pole of the pushbutton switch. Connect another wire from the other pole of the pushbutton switch to +12V via a fuse. Either use an inline 2A fuse, or connect to the downstream side of an existing fuse that serves a noncritical circuit (don't tap into the taillamps, for example). You can choose whether you want the electric release operable any time or only when the key is in the "on" or "accessory" position, depending on whether you connect to always-live +12v or key-on +12v.
If you want a switch that looks less anachronistic in a '60s-'70s car than the decidedly '80s-looking yellow or black plastic one found in cars originally equipped with electric trunk latch release, you might use a Standard #DS246:
Your electric trunk latch may have another wire to it, aside from the one used to actuate the solenoid. This second wire usually feeds a light bulb, so if you don't have a trunk light and want one, you can use this bulb to add that function. Some (all?) of the electric trunk latches equipped with lights have a built-in switch that turns the light on with the latch in the "open" position, and off in "closed", which is convenient.