Aftermarket temp gauge issue

I recently had my 86 B overheat on me so in the process of installing a new radiator to correct the issue I decided to install an aftermarket electric water temp gauge so I would know a numerical value of the operating temp. I found a wire running into the factory cluster that matched the color/stripping of the one connecting to the sender and tapped into it. I also installed a tach at that time as well. I have tapped into the dimmer wire from the dimmer switch for both gauges and that works great. Also tapped into the same ground for both. Using the same switched ign source from the PTC for both. I have no issues with the operation of the tach, but the new temp gauge is not functioning.
Let me define not functioning to mean with the sensor or switched power wire connected correctly on the gauge I receive an erratic bouncing needle. It does not give any usable information on the new gauge so I have just unplugged either the sensor or + wire from the back of it. The factory gauge still works and appears to function properly, but I'm not certain of its accuracy because there is no numerical value associated with it for comparison. I know I have a 180F thermostat but no way to validate if all is operating within the correct ranges.
I get the same results from the new gauge regardless of using the stock sender or the one that came with the new gauge. Does anyone know if the yellow wire with a white stripe that attaches to the sender is the same color code all the way to the factory cluster? Does anyone know what could be causing the erratic bouncing issue.

I have checked the voltages and don't see anything abnormal.

The aftermarket gauge should have it's own sender. I'm sure the factory gauge uses a different ohm to function than the aftermarket one. You did get a sender with the aftermarket gauge didn't you? You either need to put the aftermarket sender in place of the factory sender, or find a stand alone place for the aftermarket sender. You would still have both gauges functioning with the later suggestion.

Use the sender that came with the aftermarket gauge and run a separate wire for it.

I currently have the sender which came with the new gauge installed and yielding the same results as with the oem sender. I realize I could run an independent wire from the sender to the new gauge but was wanting/hoping to simplify the setup by using the wiring already in place.

I scoured the forum over at mazdatruckin and didn't find any helpful info about my issue. I did find a post on this site which indicated a possible problem with splitting the resistance if trying to send signal from one sender to both factory gauge and aftermarket simultaneously. I'll try and find the post again and link to it.

I know this resistance/ohm info is more in Cusser's wheelhouse so maybe he can shed some light on the subject.

Here's the link to the post talking about having the impedance split between to gauges. I think the exact reference is near the bottom of the 1st page by limequat.
http://www.mazdabscene.com/threads/19149

Where is the best location to put an aftermarket sending unit? The previous owner of my 2200 installed three gauges down where the radio used to be (oil, temp and battery). However, he never hooked up the temp gauge. The wire and sending unit is just coiled up behind the lower panel. With all of the PO's transgressions, he did (on the upside) a fantastic job of remolding the upper dash bezel to fit the radio under the HVAC controls. It appears he use JB weld on the back side to fabricate a plate then took a plastic welder to the front and sanded/painted to look like it was a factory install. I wouldn't have known what was going on till I pulled the bezel to replace light bulbs.