I'm not really sure how to provide a single answer to a question like this, so what I'll do is go through my thought process on this, and then provide a scenario at the end. Hopefully that gives you something to work with.
First: How will they learn of this?
You want them to see the consequences of their actions, which is good. How are you going to introduce this consequence to them?
One idea is something you mentioned: a new owner. Somepony more respectable (and important) than the wasteland raiders. Somepony that the party might have to deal with as part of their adventure. So while asking for information or a favor or something, they see this important stallion's new slave mare: the one they abandoned so long ago.
You may need to remind them of who she is, if it was a long time ago they might not remember. Player memory can be difficult with things like that. I'd do that in character, with her seeing them and getting upset, or angry, or trying to get their attention in some way when her owner isn't around.
That would give you a way to explain what happened. If they talk to her, she can explain how she was caught due to not having an escape vehicle, had her wings cut off, was sold into slavery, and so on.
The owner could also mention it as an anecdote, but he wouldn't know that it was the party who took the car, so it'd just be an offhand comment that they could pick up on.
Or you could have her leave that part out and just ask for help in escaping, and only tell them that stuff later. Depends on how much information you want to give them about the events of the past, but I'd probably fill in details like that.
Second: What did the raiders do to her?
You know your players better than I do, so you will be the best judge of what is acceptable at your table and what isn't. That said...
I would be VERY careful about just what you choose to use here. Since you mentioned the word "slave" several times, making her a 'hooker' would actually be rape. That's a really serious issue, and it can get both offensive and upsetting very easily if handled incorrectly. Particularly if someone in your game is or knows a victim.
I wouldn't use that at my table. Not because I don't believe my players can handle it, but because I don't believe as a DM I can treat the subject appropriately. I wouldn't want to use rape as a prop in what becomes a bad trope to motivate the party.
Every group of players is different, and it might be no problem for your group. That is something you'll have to decide on your own, I just urge careful consideration.
Here's some other options:
1. Forced labour. This can be a lot of different things, from mining, to farming, to being a maid.
2. Brainwashing (or drugging), and turn her into a guard. She seems to have combat experience from your description, and not everypony has the experience to be a good guard.
Quite a few things to do here. Once you figure out her condition, then we move on to...
Third: What do you want the party to do about it?
So, you have this mare (and the mechanic, but you focused more on the mare, so I'll do the same). She's been treated badly, and it's the fault of the fillies & gentlecolts in the party. What do you want them to do about it?
Are you trying to set it up so they want to free her and atone for what happened in the past? If so, you'll want to tune the situation that she's in so that they have a shot at doing that. If she's in a compound with five hundred earth pony guards, watched over by pegasi and with unicorn backup... the party is probably not going to try anything (we already established their level of heroic bravery). So if you intend for a rescue to be a possible option, you'll need to set the situation up such that they can do it.
That doesn't mean to make it easy. An overly easy rescue has no meaning. It means to make it something they can do, if they use their wits and/or fighting prowess.
If you're not trying to give them the option of a rescue, then you can make the situation worse.
A Possible Scenario
The players hear about a unicorn stallion that oversees some territory and is looking to hire mercenary ponies for a job in the wasteland. That job requires retrieving something hidden, while avoiding the raiders, so he needs an experienced team rather than just anypony.
When they show up to inquire about it, they're ushered into a meeting hall to see the unicorn. During the meeting he offers refreshments, and they're delivered by the mare they abandoned. She recognizes them but doesn't say anything at first.
By time the meeting is over it's too late to leave for the day, so the unicorn offers accommodation for the night. The mare comes to the party during the night under the guise of bringing them dinner, and tells them what happened. She explains that her and the mechanic were enslaved by the raiders and then sold to this unicorn, and about the injuries suffered. She asks for the help of the party to escape. Losing his new slaves will of course anger the unicorn.
That's one possible scenario. There's lots of other ways to tweak or go about it. If you do wind up going with the sexual theme, then perhaps she's actually offered to the party during the night, and they learn about it that way. (But don't do that, see above!)
And that, my little ponies, is the end of my lesson for the day. Your faithful student, Twilight Sparkle.