I have an old resin Horch 1A command car from GasoLine. I want to imrprove it, replace some pieces and add detail. Did the Horch and Steyr share similar components? Can I get a sprue of parts from the Steyr or Steyr command car kit for this?

the Horch Ia and Steyr 1500/2000A didn't share any mechanical parts other than standard German accssories that would have been used by any vehicle of their type. Yes, the steering wheel perhaps and some body details, but nothing from the chassis at all, and the bodies were similar but different too. Not a whole lot of joy there. The Steyr was developed to replace the Horch as a much cheaper and simpler vehicle, due to the expense and increased maintenance for the older Horch types.

Could I use the seats, as it looks like both had the tubular frames, rear benches, steering wheel, folded down canvas cover, and rear body plate from the Steyr kit to make the resin Horch kit more presentable?

look better. Any body parts that appear to be the same or similar enough that you can use them as is or trim them to fit, go for it. While the bodies of the Horch and Steyr were not identical, they were very similar, especially aft of the front body angle. Much of the rear body probably could be used pretty easily, plus chassis parts like the tow pintle, etc. The suspensions were totally different; the Horch was the same basic chassis as the SdKfz 222 armored car, while the Steyr 1500/2000 had a much simplified chassis with a beam rear axle on leaf springs. On the Steyr, note that the front differential was built into the lower part of the engine oil pan, so there wasn't a separate differential/gearbox for the front wheels. The top would have been close enough to use, and whatever else, especially standard parts: semaphores, tools, detail parts, etc.

Glenn:
These vehicles do not share mechanical components, as Bruce explained.
Adding some Tamiya parts, as I did, will improve the model a lot.
The Steyr's front seats are very similar to to Horch's and fit well.
I changed the front seats, headlights, tools, folded canvas top, canvas covered windshield, steering wheel. Added the rear door laders.
Take a look in the pics below, these Tamiya parts fit very well on the Gaso.line Horch, that, itself, is a good kit.
Please kit us posted with this surgery pics...
Hilton.

I was trying to improve my kit that I am trading to a wargamer. He said not to bother as wargaming doesn't require the same level of accuracy in detail as display models. I noticed that the rear plate on both vehicles looks similar and was thinking that the Tamiya piece might be used to replace the resin kit one as well. So, if I ever acquire another Horch, presuming Tamiya does not release an injection-molded one, at least I know now it can be enhanced with Tamiya pieces and PE.