i was gonna say swap the glass, but you already did it! Thats the good thing about e36 clusters, easy to swap the glass. Here is another hint for ya Steve. Get some Meguires Step 2 Car polish and a microfibre cloth. Put some polish on the cloth and rub the acrylic face of the cluster ( the glass) down in circular motions , so that you get some heat into it ( hot cloth, burned fingers LOL). It DOES polish out , and your cluster will look BRAND new.
I've done it on mine and I am happy to say that the wierd burn marks on it and scratches are 100% gone!!!!