has anyone tried replacing the headlight ballast? how did it go? how hard was it?

Okay so the ballast underneath your headlight assembly in the front only controls that lamps h i d bulb It should have nothing to do with what's going on in the back I'm assuming you're saying your brake lights in the back are intermittent the very top LEDs those are LEDs not HIDs so they have nothing to do with any ballast so you may need a tail light after you replace your tail light if you're a curious type you may take the existing tail light you took out apart and see how it all works and you'll see possibly the problem but it's a smart idea to just get a replacement tail light with the tinted brake light or not depending upon the year I like the tinted ones I put them on my lighter model vehicles they just look better but that's neither here nor there they both work the same tinted or not when the LEDs in the back start getting intermittent there's usually something wrong with that circuit board that's in the bottom of the light assembly we're talking about the tail light now You may want for your headlights since you're keeping the HID housings you might want to get the Alla LED conversion It's extremely bright if you put the same LEDs in your fog lights all the lights in the front of your car look very nice and very modern for near nothing and if you don't smack the front end into things normally the bulb should last a long time probably till you're out of the car and they look really good in the HID housings you bypass the ballasts and get your 12 volt supply from the car to run the LEDs and the shutters and you're golden your ballast is irrelevant

 

IDK how well (or for how long) cheap aftermarket ballasts work.

The big wrinkle for replacement is the bulb connector. It goes through the inside of the housing - but it doesn't fit. You have to (carefully) remove the wires from the connector and "fish" them to the bulb. Then refit the connector.

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You need to replace the light bulbs.

 

I have a 2008 Prius and one of the headlights became intermittent. I found online that it used D4R, so I bought one, and the issue peristed. So i assumed it had to be the ballast.

You have to remove the bumper to remove the headlight assembly but it isn't too bad. Youtube videos are helpful.

Unfortunately, I found out that my headlight assembly came from a different model prius unbeknownst to me before acquiring it, and my ballast was for D2R HID bulbs. I changed the HID and not the ballast, and my problems went away.

You can swap your HID bulbs from one side of the car to the other to troubleshoot if it is the ballast or the bulb.

Replacing the ballast itself is not the most difficult part of this. For me, replacing the bulb was more difficult. On the driver side you have to remove the fuse box lid or possibly the whole bumper to change the bulb because it is so tight. The passenger side is easier. But on my ancient 2008 Prius just opening those plastic covers for the bulb was really hard and I broke the tabs, perhaps because they got brittle. I had to use rubber grip pads on my hand and a lot of strength to open them, and when they close I'm not sure they are water tight any more.

Also, the stupid spring that holds the HID bulb in came off of one of my two bulb assemblies and doesn't seem to want to go back in. Something is bent or broken maybe, or I just can't reach in there. One of my bulbs as a result is basically loose in the assembly as a result because I gave up on trying to do it right after spending over an hour trying to get the spring in there. Being loose has not seemed to effect the HID several hundred miles down the road.... yet. But I won't be surprised when it shakes itself to death.

Also there is an adjustment for the bulb that can be done. Mine got buggered by switching the bulbs from left to right, but I've never been a perfectionist so I never bothered adjusting it. I'm not sure how to adjust it, only that it seems like it would be difficult and I'd never really be sure if it was optimal.