Can I input a personal Message without the navigation?

You can't, but your dealer can another Tech II only project...

My did it for free with an oil change:

If your dealer was like mine he'll need instructions:
1. Select driver information center (on Tech 2)
2. Select driver #1 or Driver #2
3. Select Driver #1 Name or Driver #2 Name
4. The Tech 2 tool displays directions for entering the Driver's Name. Read and press Enter.
5. The Tech 2 tool now displays 17 symbol numbers, with each number (32 through 255) corresponding to a symbol on the ASCII Character Table. Use the Left or Right arrows to scroll to the desired placeholder.
6. Use the Clear Entry special function button on the Tech 2 tool to clear current symbol number for that placeholder.
7. Use the numerical keypad to enter the new symbol number. Press Enter.
8. When all 17 symbol numbers are as customer desired, press the Save Name special functions button on the Tech 2 tool to store the new Dirver's Name in the Instrument Panel Cluster's memory

With or Without Nav Option. This is a Dealer Tech II ONLY project for 1998-2004 Sevilles.

Most dealers will charge for this service regardless if the car is under warranty or not. This "feature" has nothing to do with warranty service. Dealers are "obligated" to do it once per "new car sale" and that's it. And very few of the new cars were "personalized"

As I noted above, most dealers either don't know how to or don't like to do this "personalization" of the car. I had three different service advisers at my Cadillac dealer tell me that this change was not possible to do... This AFTER I showed them the owners manual that said it was.

They only did my car AFTER I printed out the step by step Tech II instructions complete with the ASCII codes of my and my wife's name.

Its a cool thing, but I would only have this done if you plan to keep the car for several years. I would also print out the detailed instructions and tuck them in the owners manual for the next owner.

At worst it is a 5 minute job. But unless you can charm them (or guilt them like I did) expect to pay 15-30 minutes worth of "diagnostic" work.