Pre. Sorry for so small amount of text - i don't like typing too much.

So you playing as taric. In my humble opinion you shouldn't go as tank or as ap-ad mix. Taric is good as ap healer-finisher. No, seriously. Last thing you need is being useless (which most taric players success at).
Primary objective of taric is stun-ap damage and heal of teammates. Secondary is Radiance ultimate. And tertiary is shatter which in my opinion gives not that much of the effort for one which usually in behind of team.

Runes talking for themselves.
Seals of potency for fast ability power at start. Surprise your enemies with powerful dazzle at beginning and maybe achieve first blood if you'll delay it and combo it with ignite.
Magic penetration from quintessences and Marks will make your primary damage ability (dazzle) very usable not for only stun, but also for kicking ***, reducing foes' life quickly.
And glyphs of replenishment will finish your build with some mana regen on earlier levels, between clarity activation and before you'll get at least tear of goddess.
Also if you finding yourself jammed with ad heroes i recommend replace insight runes on armor ones (pick on your own).

And also you can replace quintessences of insight with quintessences of potency. In case you need more ap at start (i usually doing such a thing).

Since i wasn't paying attention to mana stacking I think this build depends on clarity.
And also you can choose your spells between ignite and exhaust. (in case you need slow someone down..for some reason. Otherwise choose something instead of Cripple).

Ad and some hp.
Doran's ring. Better sell it after purchase of rod of ages. In sum-up its one-way item for ap casters. (of course you can have tome of power from the start and some bottles. But do you really need it?)
Sorcerer's shoes and void staff - great items which proven themselves over large array of ad champions as great source of magic penetration. Must-have.
Archangel's staff - your mana and ap-from mana. This one is my personal recommendation due its usability and balanced stats.
Don't underestimate Abyssal scepter - you can replace it on another ap item yes. But you truly need magic defense if you have yourself versus enemy ap champions.
Rabadon's deathcap. (i^2 - **** just got real :D). Finishing touch in your build. If you have your game last for 1 hour than you'll probably need it to move balance on your side at all. Up to 600 damage from dazzle+some from shatter. Slicing finisher. But more important that you can heal your teammates for real on 1/6-1/3 of their life-stacks (depends on their life amount ofc).
(Also good idea take rod of ages before archangel's staff in case you think your hp is too low for current situation.
Also you can replace rod of ages with Rylai's crystal scepter (if you truly sure you don't need so much mana but hp instead).

Split between dazzle and imbue spells.
Dazzle as your servant for stun and finish, Imbue team-all-time-favorite heal.
Shatter really don't need to you since you'd better not let your face into direct fight with ad champions.
Radiance IMHO only usable for turret smashing, running away chase scenes when you have speed (its healing effect is ********, its attack addition is pathetic most of the times and its mana consumption just pain in the ***), and very rarely killing someone. Cut it out - better do kills with dazzle (and maybe a bit with shatter).

And its all goes. Strangely good =). Of course some of folks which saw more powerful Tarics with ad or tanks will yell at you...but do we really need to talk about 13-year-old douches which are 30% of LoL players? I think not.

And in the summary i also want to ask you guys, what do you think and what do you like to improve in this build?