1969 on up fuse boxes have a provision for 
additonal Accessory power lines using an extra
fused three prong plug.  The buss extender 
bolts into the fuse box with a carriage bolt.  The
factory buss extender is on the left
and the manufactured version on the
right.  A two year warrany is provided.

1971 through 1973 center gauge clusters
come with dashes with tachometers and in
1971 as a special order with a standard
dash cluster.  These harnesses are
available matching the factory wiring. 
The only change needed are the lamp
sockets, which now use a two wire
affair and an 1895 bulb instead of the
194 bulb.  A two year warranty is
provided

1969 on up use a flexible circuit card that mates to the underdash harness and supplies
various signals to the dash cluster.  These cards' copper traces break easily and
many customers complain about the reliability of reproduction circuit cards.  The dash
clusters can be hard-wired much like earlier Mustang dash clusters for better reliaibility
and electrical continuity.  This change does not increase dash bulb intensity, unfortunately.
The harness comes with an additional length to better access the back of the dash
cluster for connecting the new multipin connector and general trouble-shooting.  Hard-
wiring does require a new dash cluster connector.  The existing connector can be
left in place or removed.  To use this service, the underdash harness must be sent in
and only in unusual cases will the dash cluster itself needs to be shipped.  Each harness
is checked by activating all lamps, gauges, and indicator lights on the dash cluster.  A
two year warranty is provided.

Here is an example of a 1971 non-tach dash cluster
that is now hard-wired.

A popular option for many is to replace the factory dash cluster with an aftermarket     
dash cluster, often with additional gauges and tachometers.  Some clusters come
with gauges already installed and some clusters allow the customer to buy gauges
separately.  In many cases, it is left to the consumer to wire the cluster to the
underdash harness.  With shipment of the underdash harness along with the dash
cluster filled with gagues (and instructions for wiring the gauges), a complete dash
cluster harness can be wires with an additional length to better access the back of
the cluster for installation and possible trouble-shooting.  Each harness is checked
for proper operation on the bench by activating all lamps, gauges, and indicator
lamps.  A two year warranty is provided.

Here is an example of a 1969 aftermarket cluster
with Autometer gauges that is now hard-wired.