I'm having the same issue. The <sirenSettings value="29" /> line in carvariations.meta stops everything from flashing. If I put it on setting 1 the lights flash but one of the bars is way out in front of the front windscreen. Any ideas?
Using OpenIV, open police4.ytd and export the unmarked texture to your desktop, then rename it to vicz_sign_1. Go back to OpenIV, open police4.ytd again and "replace" the existing vicz_sign_1 with the renamed livery on your desktop. Then delete all the other liveries you don't want and you should hopefully just be left with the unmarked one.