Okay, I have been running the numbers and field testing this build and as far as I'm aware, it works well. I'm not going in depth and I'm not about to tell you that this is how to play Trundle, this is just how I do it.

What you'll find is the way I like to use his skills and the way I like to build items on the ugly fellow.

I find Trundle to be a great hit and run character, especially while making use of his three main abilities. However, his skills are for you to use as you please.

Rabid Bite is your spam skill. You use it, reduce the victim's attack damage while increasing your own. I've been thinking of trying to use this on a 'soak' build for Trundle while using a Frozen Heart to reduce a carry's damage output as far as possible. However, on this build, it is great to spam out for that extra damage and 'increased attack speed'.

Contaminate is your skill. It is your stage, where you shine the most. Reduced CC effects, increased attack speed, and the part I love most... The increase in movement speed. It allows you to make a quick run at them, or a hasty escape if it comes down to that. It is great for pushing your damage output just that little bit higher.

Pillar of Filth is useful for more than just blocking champions, dropped it in the middle of the enemy team allows your tank or initiator to get in there while they're grouped. It can also be used to disrupt channels, if dropped correctly it will force a move action (if this has not been changed in recent patching). Early game, bushes and Pillar of Filth will be your friend if you're trying to drop in for an early gank.

Agony, in a team fight I will save this for their soak/tank as it removes a fair chunk of their defenses while bolstering your own. The damage isn't what's important, the damage mitigation is. It allows you to remain in the team fight for a much larger duration. Assuming you don't get focused.

Okay, reasoning behind the item build.
Wriggles Lantern is a personal favorite of mine. It has great farming capabilities and allows Trundle to have a constant eye out for any who might try and gank you early game. If all goes well, you should be able to rush this item fairly quickly. The lifesteal and ability to insta-jib most minions allows great self-healing on Trundle when mixed with his passive.

Merc Treads, generally you will run into teams with heavy CC effect and mixed with his W skill, they will mean nothing to you. Plus, the magic resist will still put you 5 up on those who buy Sorc Boots.

Frozen Mallet and Phantom dancer should explain themselves. It is everything a carry/anti-carry could ever wish for. Health, attack damage (even if not much), an on-hit slow, movement speed, crit chance and attack speed. All of these work extremely well on Trundle. One hit and there should be no way for them to get away from you, unless they have Flash or -insert x amount of targetless charge or teleport skills here-. Being able to keep a target where you want them is something that Trundle excels at. These items just make it that much easier.

Now, towards late game, your opponents will have 70-90 armor on average. Black Cleaver and Stark's Fervor help counter this, it makes you good at forcing a tank to your level of defenses in conjunction with your ult. The Cleaver used to be better with giving higher attack damage and no attack speed, but what can you do? It's Last Whisper 2.0... Stark's Fervor is also just a personal favorite. The attack speed and lifesteal combo with armor reduction makes this aura-baring item worthy for any carry/anti-carry to wield.

I'm not saying that this build will win you every game, I just want to see what people make of my build. I don't see anyone using it. I see the Phantom Dancer/Frozen Mallet combo everywhere, but that's just because it's common sense on physical characters. So, to whoever is daring enough to play Trundle, please try this build and tell me how it goes. :D