Hi mate! When you say the grille, I'm assuming you mean the plastic black honeycombe center section in between the front fog lights?
I replaced one on a 156 a while back, it was quite a simple job to be honest. (Assuming that your grille is the same as the 1998 156 grille)
Firstly, buy the grille off of www.shop4parts.co.uk
,it was about £22 including postage.
Secondly, and most importantly, you do NOT have to take the bumper off if you do it the way I did.
Here's a quick step-by-step guide for you for once you have removed the tatty old grille. (You can just snap this out)
1) Make sure all of the screws are removed from the bumper holes that the grill slots into. (I had one little bleeder left which I had to pull out!)
2) Remove the OS and NS fog lamp grill assembly's. (These need to be removed as when you put them back on they will cover up a lip on the grill and hold them in place better)
3) Get 2 black cable ties and loop them around and out of the centre bumper grill post. (This is a 6" maximum bar that runs from the top of the hole to the bottom of the hole where the grill should be)
4) Line the grill up to the bumper (Where it will be fitting) and pull the 2x cable ties through a section of the top of the grill and a section of the bottom of the grill.
5) Fasten slightly both cable ties so that the grill is held loose in place
6)Line the grill up properly and push it into the bumper so that all of the lower stud pins and upper stud pins go into the bumper holes.
7) Pull the cable ties now as hard as you can so that they are as tight as possible and cut any excess tie you have left off, then slide the tie around so that the cable tie catch is not on show.
8) Now re-install both the OS and NS fog lamp grill assembly's so that they cover the left and right lips of the grill overhang.
9) Stand back and admire your best efforts,
If you take a look here:
(Should take you to Page 6???)
You can see a photo how-to I did as well mate,
It really is a VERY simple job. All you need is a couple of black cable ties, a pair of scissors and a cross head screw driver to remove the fog lamp grille sections,
Let me know if that makes sense to you bud as I can sometimes get confusing,
I know that the cable tie method sounds crass but SERIOUSLY, you cannot see them at all and they hold the grille on perfectly along with the fog lamp grills, you can see from the pictures how good it looks after anyway. The main issue (As seen further down on Page 6 of that link) was getting the cable ties looped through the bar and grill nice and evenly, I re-did it as I wasn't happy with the honeycombe's they went through first off,
Good luck mate and let me know how you get on,
PS: You also should get a free shop 4 parts key ring with every order,
EDIT: Just realized you are in Australia mate, so not sure if you can still get delivery from Shop 4 Parts. HOWEVER, you should be able to source a grille relatively easily and cheaply I should have thought over there,