I got an under the counter LED light (uses 6 AA batteries) BUT it lasts no time so I also have a few 12 V power units for modems, switches and stuff. Can I wire the 12 volt switching power unit into the 9 volt LED light with no overheating, sparks, etc. ???
or do I need to get a 9 volt plug in power unit?

 

Welcome to AAC!

Without more information, no one can say.

We need to know how many LEDs and how they're are wired. Are there any current limiting resistors? Are they driven by a current source (not likely because that adds cost)?

The safest thing to do is to use a 9V adapter. One hack would be to insert a few diodes in series with the 12V adapter to drop the voltage close to 9V.

It would be best if you made a schematic to show us how things are connected. Model numbers probably won't be helpful.

 

Don't know what your skill level is, but if it were me I wouldn't use the 12 adapter without placing a series resistor in the circuit. What value? You would have the measure the current (I) in the circuit when using batteries. Then the effective resistance (R) of the LED lamp would be R = 6/I. Use a value of resistance equal or close to that, since you're powering with 12v rather than 6. Also, what wattage (P) resistor? That would be P = 6*I, so use at least twice that for wattage. I'll guess your resistor will wind up being close to 56 ohms, 2 watt. (That's a guess, assuming your batteries last around 10 hrs or so.)

Edit: Misread your post. Thought your batteries were 6v, not 9. So use 9 up there rather than 6.

 

You start with 100R resistor. Measure the voltage across the leads going to the strip. Put another 100R across the 100R and measure the voltage. Keep doing this until the voltage is 9v.