Oh jeez..  I haven't messed with it in a long time..  I *THINK* it's
Auto Tap and I'm using their cable.  I really only used it as a basis
to develop a "glass" dashboard for my truck to get rid of the steam
guage instruments but lost interest in the project when I took up
flying helicopters..  ehhehe

Unless you have a "drive-by-wire" car (read expensive); even an old 386
laptop can "read the serial data in real time" (actually even less but a
386 was where laptop color displays became common. You rreeaaalllyy
need/want a color display for this!).

There's one reason why older laptops go damned cheap; dead batteries.
Often the replacement is more than what the 'puter is worth.
Sometimes there is no source for replacements.

For a diags/performance meterputer that's used only in the car or
plugged into 120VAC; that's no biggy.

If the laptop says 14-18 volts for AC adapter; it will probably run on
"12"volts quite well. The extra volts are "charging overhead". Dead
batteries don't need no steenking charges.  Go to RatSnack and get one
of those ciggyliter power cords with the assorted power plug ends. Find
the right one , test it and heatshrink otr tape it (glue?)so it doesn't
fall off.  You now have a "carputer" . Just remember to shut Windows
down before you pull the plug.

As for the dead battery:

If it slides into a pocket or fits under a cover, just pull it out. You
may want to do something ro fill up the hole... get creative.

If it's "kinda like part of the bottom" and won't sit well if removed;
you can carefully crack the seams; gut the cells out; glue it back
together; and put it back. If you know what you are doing; you *could*
hardwire a ciggyliter cord to the terminals of the battery. Caution;
doing this wrong could result in ugly smells and funny sounds and a
slagged laptop. Electronics stuff works far better when you don't let
the smoke out.