From Oldsmobile list server... Andrew C. Green explains how to re-wire front side markers to blink with turn signals

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Disclaimer: These instructions are for general information only.
Any modifications you do to your vehicle are at your own risk.
No warranty is expressed or implied!
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Summary
-------
This procedure will enable the front side marker lights of any car
to blink _with_ the turn signals (if running lights are off) or to
_alternate_ with the turn signals (if running lights are on). Normal
operation of the marker lights is not affected. The procedure works
on any car with parking and turn signal lamps at the front, even if
the turn signals are not dual-filament (#1157/1157A/1157NA) bulbs,
such as on Japanese cars.

Restrictions
------------
The marker lamps must not be single-wire designs which rely on the
car body for grounding. If the marker lamp has two wires connected
(power & ground), the procedure will work. 

History
-------
This wiring alteration reproduces the standard design first used (and
presumably patented) by General Motors in 1971. A similar approach was
used by Ford for one year in 1970, but that design blinked both front
and rear markers and was apparently abandoned after the 1970 production
cycle. Following the apparent expiration of the patent in 1986 or there-
abouts, other manufacturers began to also use this wiring scheme.

Basic Design Premise
--------------------
Wiring to the marker lamp is rearranged such that the lamp bridges the
parking and turn signal circuits, drawing power from whichever circuit
is "hot" and grounding through whichever circuit is dead.

Tools Required
--------------
12V test light with needle probe
Soldering iron or 3M Quicksplice connectors, or equivalent
Short length of 14-gauge stranded wire (possibly)

Instructions
------------
The whole job involves only re-routing one wire on each side. You just
need to check carefully to ensure that you've identified the right ones.

1) Switch on the vehicle's parking lights and 4-way emergency flashers.
   You don't need to run the engine during this test unless the battery
   is severely low.

2) Using the test light on the wiring going into each turn signal, identify
   the wire carrying the blinking turn-signal power.

3) Using the test light on the wiring going into each front side marker
   lamp, identify the ground wire (the one that's _not_ hot).

4) Switch the vehicle lights off. You don't need to disconnect the battery.

5) In the next step, you're going to snip the ground wire coming from the 
   marker lamp and connect that wire to the turn signal's power feed. Work 
   out the best way to snake the marker lamp ground wire around to a point 
   where you can tap it into the turn signal's power feed wire, using an 
   extension of 14-gauge wire if necessary. Pay attention to what cavities
   or openings might be filled when you reinstall the light assemblies, if
   you removed them previously. You don't want to pinch the rerouted wires.

Note:	Don't get confused here. The idea is to run the ground wire
	FROM the marker lamp TO the turn signal. Currently it runs
	from the marker lamp into the wiring harness and grounds
	somewhere. You're _not_ trying to connect the harness ground
	to the turn signal!

6) Cut the ground wire coming from the marker lamp, giving yourself as much
   wire length as possible from the marker lamp to the cut point. Tape the 
   cut end of the ground wire that goes into the vehicle harness and stuff
   it out of the way.

7) Connect the ground wire leading from the marker lamp to the hot lead
   going to the turn signal on that side. Tape or otherwise weatherproof
   the connection, and you're done.

Notes:

	1) To tap into the turn signal's hot lead, you can melt a short
 	   length of the insulation with the soldering iron, let cool,
	   then pull the melted insulation off with your fingers. Wrap
	   the ground lead from the marker lamp around the exposed wires,
	   solder properly and tape up. Try to only use the Quicksplice
	   connectors if space is a problem.
 
	Ground from Marker Lamp ===\
		 		    \
	Wiring harness <============-\/\/\/\/-======> To front turn signal
	                             ^^^^^^^^
	                        wrap & solder here

	2) Most markers use a little #194 or #194A bulb. These have a lot
	   of work to do when operating with turn signals, so pop in new
	   ones at this time. Wipe Vaseline on the base contacts for good
	   connections.

Andrew C. Green            (312) 266-4431
Frame Technology Corporation
Advanced Product Services
441 W. Huron               Internet: acg@frame.com
Chicago, IL  60610-3498    FAX: (312) 266-4473