Jim,
The picture above shows the pins of a SPDT relay which fits into a base that has a wire out for each of the pins. Pin 30 gets your common 12V hot wire which will power the two circuits; Pin 87 will be your 12V+ to your brake lights; Pin 87a is the 12V+ to your lock up converter; Pin 85 goes to ground; and Pin 86 is your 12V+ switched control wire from your mechanical brake light switch which gets it's power from your fuse block "Brake Lights" wire. Wire that fused supply to one side of your brake light switch, then take the piece you cut off of that wire and hook it from the opposite side of the switch running to Pin 86.

Here's a link to The 12Volt, which is an excellent source of understanding basic direct current circuits.
Single Pole Double Throw (SPDT), Single Pole Single Throw (SPST) Automotive Relays

Hope that helps.