frequency of data wires in a vehicle?

The 20mhz scope should work fine, but a multimeter will do the trick for much less.

With a decent meter you can measure either voltage or frequency of the signal you are attached to.  A data bus won't stay at 12vdc consistently and a data wire will show some sort of frequency while its communicating.

Set the meter to frequency and watch it for a second.  Some Fluke meters tend to read 60hz even if there isn't a frequency present so watch out for that.

You won't be able to see the actual data, or read it in any way, and chances are the frequency won't ever stablize as the bit information will most likely be constantly changing resulting in different frequencies.

Like I said though, if you are looking for power wires in a vehicle just check to make sure you have battery power.  Data signals will usually read really low with a DC meter, around 2-3vdc (not 100% of the time, but mostly).  It should be impossible to accidently tag a 12vdc wire that is a data wire, IF you meter the wires first.

What exactly are you trying to do?  What signals do you need to find?  Why are you so paranoid about tapping in to a data wire?

So what kind of signals are you looking for????

Data wires are typically smaller then power wires.  I don't know exactly what you are trying to do, but there are "other" easier ways to tell if you have the correct wire.

The only time I've ever need an o'scope on a car is to actually decode the data being sent.

If your wire shows +12v DC constant or +12v DC switched, there's no data.  period.  They do not multiplex data on 12v DC wires.  Computer data systems on cars are based on 5v format.  Shove 12v DC onto a data wire and goodnight Irene.  So, if you test the wire and it' shows 12v DC, you are fine.  When the wire is multiplexed to control other features in the cars, such as Mopar lights and door locks, you'll see three different DC votages depending on what you are trying to control.

Bottom line is that you will not see +12V DC on a data wire.

Yeah, like I said if a wire has battery voltage on it constantly it can't have data on it.  Throw a volt meter on it and watch it for 30 or even 60 seconds - if it stays constant then you are good to go.   Another good rule of thumb is to never use any wire smaller then 18awg wire for power - that right there should eliminate your issue as most data wires are 20awg or smaller.

Also, if you just need switched and ignition I would always look for the steering column wires or the ECU power wires - in both cases the wires will be larger then most other wires in the area.