My,s wont go out replaced left taillight all new,checked everything else all good,order new bulb out relay.Weres this relay?

under dash left side close to driver
red-orange can about 2.5 inches diam length width
has a gazillion contact terminal pins so it needs persuasion to get disconnected
maybe thats where your punctuation and sentence structure are too

in a '91 with knee bolster you might get there aftr you pull off the bolster but im not sure
if not then you gotta pull the cluster and rech in that way
anyway be sure to pull the battery ground cable 1st or you might fire off the airbag and give yourself a bad day.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.

With the knee bolster out, you will find the orange can just above the lip of the dash, right above your left kneecap. Not cheap if it has failed. One part of it detects low beam headlight failure. Unfortunately, when that circuit fails, you have no low beams at all, which is what happened to me a few weeks ago. I bypassed the low beam power around the bulb out sensor. I can tell when a headlight burns out without the help of that sensor.

It only comes on wind I turn on my head lights/park? I do have low/high beams.

As I recall, there were three sets of windings in the bulb-out relay.

One for the low beam headlignts, one for the tail lights, and one for the brake lights. Any one of the three may fail.

Before suspecting the relay, are you sure that the wattage is correct in all your tail lights? I had the wrong wattage in the two lower brake light bulbs in my wife's car for a while, due to a confusing listing in the reference book at the auto parts store. That caused the current flow to be unbalanced between the lower and upper bulbs, and caused the bulb-out warning to light.

Yes - those are the circuits it checks.
It ignores the front corner lights, backups and fogs.
It looks for imbalance left vs. right in those 3 circuits.

Mismatched bulbs left/right will trip it to give a bulb-failure warning (different brands, ages, etc)
Poor contact or weak ground on either side will also trip it.
Basically it detects the current draw; anything that influences the amount of current passing in left vs. right can trip the circuit and give you a "bulb out" warning light.

Of course when the !$#(P*&! sensor/relay unit fails internally, the circuit in question goes out to lunch.

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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.