Get a lawyer. I doubt you will get it on your own. You can try the open records law, but the police will reply that it is an open criminal investigation. It will also tip off the officer to your defense before he files the charges.
What is more important is that the tape be preserved. Dash cam tapes have a habit of being unavailable when the pre trial hearing comes to court.
Get a good defense lawyer. Have your lawyer write a letter to make the DA and the police preserve the exculpatory tape. It is exculpatory evidence.
Our firm has had charges dismissed because the dash camera did not show the bad driving the officer testified that he saw. An illegal stop will result in suppressed evidence. If the officer says he stopped you for crossing the line and the tape shows you did not, the prosecution has problems.
Tapes also let the jury see how you really did on the roadside coordination tests. Often that is better than the officer's scoring of the "test".
You were arrested for DUI. Get a good lawyer sooner, rather than later.
I do not think you will be successful as a layperson asking the police for their investigation information.
Hire an attorney to establish your defense to the charges. And, no matter what the film footage shows, the field sobriety tests are going to come into evidence unless your attorney can move to suppress them. So, use an attorney if you know for sure your test results were cooked up.
If not, then this is all an academic exercise, right?
I agree with Mr. Feeney and Mr. Brinkmeier that you need an attorney for the DUI charge. Going this alone would be a very bad move. I disagree that it is difficult to get a copy of the Dash Cam footage. Where you case is procedurally is not considered an ongoing investigation. Two things need to be done to get the footage. First you need to notify the police that you want the footage preserved you need to be specific about arresting officer, time date location etc. This notice should be in writing and sent fax or certified mail or some other way to prove delivery of the notice. Second the footage should be requested in criminal discovery. You may also want to demand preservation of the radio tapes. That request may have to go through the County rather than the local PD or State Police. Yet another reason to retain counsel.
Now what I said is easier said than done and that is why you need an attorney representing you in this matter. Even if you can get the footage on your own you probably won’t' know what to do with it or how to present it so that it can be used in your favor. Having counsel assist you with this will not add extra cost to the case. One final caveat many police departments in PA don't use dash cams.