W126 intermittent partial cluster lights
 

On my 89 560SEL occassionally the right hand side bulb that illuminates the rev counter doesn't come on.

Tapping on the front of the cluster a few times causes the bulb to come on.

I am about to pull the cluster and replace both bulbs but is there anything else worth checking whilst I have it apart?

Cheers
Matt.

NO, you are on the right path of what to check

Consider replacing all the bulbs or at least cleaning the bulbs with Windex or alcohol. Pull the gauge cluster dimmer (one screw and some prying) and clean the contacts or fit a new one. Don't forget to clock the dimmer knob when you reinstal the dimmer.

Sixto
95 S420
87 300SDL

One thing that I noticed on the cluster of my 126, is that the white plastic portion that the bulb sits in, had become a bit yellowed over time.
I got out my model paints (I was pleasantly surprised they hadn't dried up after all these years), and using some white paint, touched up the yellowed plastic. Not sure really exactly how much it helped...but, I am sure that it didn't hurt either.