Dual battery question

[uncle mitchie]

Have just purchased 1987 GMC surburban, has dual batteries .One battery is there but are only cut wires for the aux battery. Should be easy right, but I need to be sure about wiring before proceeding. theres a red wire with a fusable link to a solnoid mounted on fender with red wire to aux power point next to it, a ground wire and a small black wire which runs to the fuse panel under the dash.
Need advice about running aux battery, what keeps it charged?, is this the right wiring?
Thanks
Mitch