Have searched thorugh the archives & didn't find my question addressed...

I need to replace the lower of the two horns on my '94 E320 Cabriolet as it just makes a "tup" sound when I try it--the upper horn is fine. Which frequency is the [physically] lower horn?

Established that the problem is the horn and not wiring. I reversed the two leads from the working [upper] horn down to the lower horn & the lower still didn't work. Also verified the wiring by testing the lower 2 leads on the upper horn & it still worked fine.

I've seen units for sale listing the 500Hz 433Hz combos & also a 433Hz 300Hz combos. But if I only buy one horn, which should I buy?

If you remove the broken horn, the frequency should be marked somewhere on it. (Maybe you can read it without taking it off) Just buy the same thing & replace. Be sure to tape the leads while the horn is off.
Good luck

jaoshea, you were quite right. On the back of the defective horn is listed 405Hz. In fact, on the back of the working horn is 485Hz. So, I ordered the lower frequency horn from Fastlane yesterday & it's already out-for-delivery today! That's great service!!!