ground goes through vehicle chasis.
[RELAY] is any existing relay in the stock system.
[SWITCH] is the current controlling switch in the stock system,
  wether computerized of manual, or mechanical.
[I2C SWITCH] is the new, computer controlled switch.
[SPDT RELAY] routes the top pin through to output when not energized.
[LIGHT] is whatever light or device I'm controlling.

                                                    +--------------+-------------------+
                           |                        +              |     /MASTER\   [BATTERY]
                           |                        | +--------------+---\SWITCH/------+
   +--[RELAY]-+-[SWITCH]-------------------------+  | |            | |
   |          |            |                     |  | |            | |
   |          |            |                [SPDT RELAY]---+    (to other
   |          |            |                     |         |   xPDT relays)
   |          +---------------[I2C SWITCH]-------+         |
   |                       |                               |
BATTERY                    |                               |
   -(ground)               |                               |
   |                       |                               |
   +-----------[LIGHT]-------------------------------------+
                           |
                           |

Anything to the left of the centerling must remain.  The only changes should
be the splicing in of a wire between [RELAY] and [SWITCH], and the insertion
of [SPDT RELAY] in the wire between [SWITCH] and [LIGHT].  This effectively
isolates the [SWITCH], and allows the computer (via the [I2C SWITCH]) to
have complete control over the [LIGHT] when the [SPDT RELAY] coil is
energized.